Optoelectronic device

ABSTRACT

The invention provides an optoelectronic device comprising a photoactive region, which photoactive region comprises: an n-type region comprising at least one n-type layer; a p- type region comprising at least one p-type layer; and, disposed between the n-type region and the p-type region: a layer of a perovskite semiconductor without open porosity. The perovskite semiconductor is generally light-absorbing. In some embodiments, disposed between the n-type region and the p-type region is: (i) a first layer which comprises a scaffold material, which is typically porous, and a perovskite semiconductor, which is typically disposed in pores of the scaffold material; and (ii) a capping layer dis -posed on said first layer, which capping layer is said layer of a perovskite semiconductor without open porosity, wherein the perovskite semiconductor in the capping layer is in contact with the perovskite semiconductor in the first layer. The layer of the perovskite semiconductor without open porosity (which may be said capping layer) typically forms a planar heterojunction with the n-type region or the p-type region. The invention also provides processes for producing such optoelectronic devices which typically involve solution deposition or vapour deposition of the perovskite. In one embodiment, the process is a low temperature process; for instance, the entire process may be performed at a temperature or temperatures not exceeding 150° C.

FIELD OF THE INVENTION

The invention relates to an optoelectronic device, and in particular toa flat junction optoelectronic device. The invention also relates to aprocess for producing such an optoelectronic device.

BACKGROUND TO THE INVENTION

Thin-film photovoltaics are a promising alternative to theirmonocrystalline counterparts owing to their high-efficiency, comparablestability and potentially lower manufacturing cost. The mostwidely-studied thin-film materials currently under investigation forphotovoltaic applications include the compound semiconductors CdTe [X.Wu, Solar Energy, vol. 77, p.803, 2004], CuIn_(1-x)Ga_(x)Se₂ (CIGS)[Chirila et al., Nature Materials, vol. 10, p. 857, 2011], Cu₂ZnSnS₄(CZTS) [D. Barkhouse et al., Progress in Photovoltaics, vol. 20, p. 6,2012]; dye-sensitized solar cells [A. Yalla et al., Science, vol. 334,p. 629, 2011]; and organic semiconductor solar cells [Y. Liang et al.,Advanced Energy Materials, vol. 22, p. E135, 2010]. Inorganic compoundsemiconductors which comprise high-efficiency solar cells are typicallyfabricated using expensive vacuum-based deposition although recentroutes towards solution processing of CIGS and CZTS have exhibited highefficiency devices [M. Graetzel at al., Nature, vol. 488, p. 304, 2012].Dye-sensitized and organic solar cells with lower record efficienciesare typically fabricated with solution-based deposition procedures butsuffer from poor long-term stability. In addition, the relatively lowproduction capacity of tellurium and indium makes CdTe and CIGSpotentially commercially unattractive.

Perovskites [D. Mitzi et al., Science, vol. 267, p. 1473, 1995] are analternative family of semiconductor materials which have beeninvestigated for device applications [D. Mitzi at al., IBM Journal ofResearch and Development, vol. 45, p. 29, 2001]. For photovoltaics,perovskites have been used as the sensitizer in liquid electrolytephotoelectrochemical cells [J.-H. Im et al., Nanoscale, vol. 3, p. 4088,2011; A. Kojima et al., Journal of the American Chemical Society, vol.131, p. 6050, 2009], although in this previously reported electrolytesystem, the perovskite absorbers decayed rapidly and the solar cellsdropped in performance after only 10 minutes. Perovskites have also beenused in solid-state photoelectrochemical cells [H.-S. Kim et al.,Scientific Reports, doi:10.1038/srep00591; A. Kojima et al., ECS MeetingAbstracts, vol. MA2007-02, p. 352, 2007] and as the hole transporter insolid-state dye-sensitized solar cells [I. Chung, Nature, vol. 485, p.486, 2012]. The main operating principle of sensitized solar cells, isthat the role of light absorption, and charge transport are separatedinto different materials in the solar cell. This enables light absorbingmaterials, which would generate charge inefficiently if light was shoneon a solid film of the material, to operate very efficiently in asensitized solar cell. Hence, since there are examples of perovskitesemployed as sensitizers in meso-structured solar cells, or ashole-transporters in dye-sensitized solar cells, but no reports of solidfilms of perovskites operating efficiently in solar cells, it would bereasonable to assume that perovskites are not an ideal family ofmaterials to employ as solid thin films in thin film photovoltaics.

SUMMARY OF THE INVENTION

The invention provides optoelectronic devices having a thin film of alight-absorbing or light-emitting perovskite disposed between n-type(electron conducting) and p-type (hole conducting) layers. The inventorshave unexpectedly found that good device efficiencies can be obtained byusing a compact thin film of the photoactive perovskite, as opposed tothe requirement of a mesoporous composite. Whilst an open porousperovskite structure could typically be infiltrated with a p- or n- typematerial to form a bulk heterojunction with that material, the denseperovskite layer employed in the present invention will generally form aplanar heterojunction with the p-type layer and/or the n-type layer.

The perovskites employed in the optoelectronic devices of the inventionare attractive for optoelectronic device applications because they canbe formed from earth-abundant elements by both solution and vacuumprocessing, have tunable band structures (and therefore optical andelectronic properties), and can be stable in atmospheric conditions. Theinventors have shown that the photoactive perovskite film may be grownon a thin scaffold or seed layer, or in the absence of such scaffold, bysolution deposition. Devices incorporating the thin seed layer can beprocessed entirely at temperatures not exceeding 150° C., which isimportant for reducing the cost of manufacturing, and for enablingprocessing on plastic substrates to provide flexible devices, and alsofor enabling processing on top of other layers to enable the productionof tandem and multi-junction devices. The perovskite thin film can alsousefully be formed by evaporation from a bulk powder or byco-evaporation of perovskite precursor compounds.

Accordingly, the invention provides an optoelectronic device comprisinga photoactive region, which photoactive region comprises:

-   an n-type region comprising at least one n-type layer;-   a p-type region comprising at least one p-type layer; and, disposed    between the n-type region and the p-type region:-   a layer of a perovskite semiconductor without open porosity.

Typically, the optoelectronic device is a photovoltaic device.

Alternatively, the optoelectronic device may be other than aphotovoltaic device. The optoelectronic device may for instance be alight-emitting device.

In some embodiments, the photoactive region comprises:

-   said n-type region;-   said p-type region; and, disposed between the n-type region and the    p-type region:    -   (i) a first layer which comprises a scaffold material and a        perovskite semiconductor; and    -   (ii) a capping layer disposed on said first layer, which capping        layer is said layer of a perovskite semiconductor without open        porosity,-   wherein the perovskite semiconductor in the capping layer is in    contact with the perovskite semiconductor in the first layer.

In another aspect, the invention provides a process for producing anoptoelectronic device comprising a photoactive region, which photoactiveregion comprises:

-   an n-type region comprising at least one n-type layer;-   a p-type region comprising at least one p-type layer; and, disposed    between the n-type region and the p-type region:-   a layer of a perovskite semiconductor without open porosity,

which process comprises:

-   (a) providing a first region;-   (b) disposing a second region on the first region, which second    region comprises a layer of a perovskite semiconductor without open    porosity; and-   (c) disposing a third region on the second region,

wherein:

-   the first region is an n-type region comprising at least one n-type    layer and the third region is a p-type region comprising at least    one p-type layer; or-   the first region is a p-type region comprising at least one p-type    layer and the third region is an n-type region comprising at least    one n-type layer.

Typically, the process of the invention is for producing a photovoltaicdevice comprising said photoactive region.

Alternatively, the process may be used to produce an optoelectronicdevice other than a photovoltaic device, which optoelectronic devicecomprises said photoactive region. The process may for instance be usedto produce a light-emitting device comprising said photoactive region.

In some embodiments of the process of the invention the photoactiveregion comprises: said n-type region; said p-type region; and, disposedbetween the n-type region and the p-type region:

-   (i) a first layer which comprises a scaffold material and a    perovskite semiconductor; and-   (ii) a capping layer disposed on said first layer, which capping    layer is said layer of a perovskite semiconductor without open    porosity, wherein the perovskite semiconductor in the capping layer    is in contact with the perovskite semiconductor in the first layer.

In such embodiments, the process of the invention comprises:

-   (a) providing said first region;-   (b) disposing said second region on the first region, wherein the    second region comprises:    -   (i) a first layer which comprises a scaffold material and a        perovskite semiconductor; and    -   (ii) a capping layer on said first layer, which capping layer is        said layer of a perovskite semiconductor without open porosity,        wherein the perovskite semiconductor in the capping layer is in        contact with the perovskite semiconductor in the first layer;        and-   (c) disposing said third region on the second region.

The invention further provides an optoelectronic device which isobtainable by the process of the invention for producing anoptoelectronic device.

Typically, the optoelectronic device is a photovoltaic device.

Alternatively, the optoelectronic device may be other than aphotovoltaic device. The optoelectronic device may for instance be alight-emitting device.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 provides schematic illustrations of: (a) the generic structure ofan embodiment of the optoelectronic device of the present invention; and(b) the photovoltaic cells exemplified herein (variations where the thinAl₂O₃ or TiO₂ layer is omitted are also investigated). At least one ofthe metallic electrodes is semi-transparent across the visible to nearInfrared region of the solar spectrum. Semi-transparent is with atransparency of typically 80%, and ranging from 40 to 90%.

FIG. 2 shows (a) XRD spectra of perovskite films grown on each of theunderlayer variations investigated in the Examples hereinbelow; and (b)an XRD spectrum of perovskite formed by evaporation.

FIG. 3 shows normalised UV-vis spectra of perovskite films grown on eachof the underlayer variations investigated in the Examples hereinbelow.

FIG. 4 shows representative J-V characteristics of devices with thefollowing variations: (a) HT B—Al₂O₃, (b) HT B—TiO₂, (c) HT Al₂O₃, (d)HT TiO₂, (e) HT C, (f) LT Al₂O₃, (g) LT TiO₂, (h) LT C, and (i)Evaporated.

FIG. 5 shows SEM micrographs of solar cell cross-sections with thefollowing variations: (a) HT B—Al₂O₃, (b) HT B—TiO₂, (c) HT Al₂O₃, (d)HT TiO₂, (e) HT C, and (f) LT C.

FIG. 6 shows top view planar SEM micrographs of substrate treatmentswith the following variations: (a) HT B—Al₂O₃, (b) HT B—TiO₂, (c) HTAl₂O₃, (d) HT TiO₂, (e) HT C, and (f) LT C.

FIG. 7 shows planar SEM micrographs of substrate treatments with thefollowing variations: (a) LT Al₂O₃, and (b) LT TiO₂.

FIG. 8 shows a device schematic for inverted p-i-n type thin film solarcells.

FIG. 9 shows steady state photoluminescence spectra for bi-layers of aperovskite absorber upon a) p-type layers and b) n-type layers. Theemission is centred on the photoluminescence peak of the perovskiteabsorber CH₃NH₃PbI_(3-x)Cl_(x).

FIG. 10 shows a) SEM cross-sectional image of the optimized inverteddevice configuration. Scale bar represents 250 nm. The different layershave been coloured with the colour scheme of the device schematicdrawing shown in b).

FIG. 11 shows an SEM top view of substrates after the deposition ofPEDOT:PSS followed by the perovskite layer for substrates containing a)and c) PEDOT:PSS annealed at 150° C. for 20 min and b) and d) PEDOT:PSScrosslinked with a 0.25 M aqueous solution of FeCl₃. Scale bars in a)and c) correspond to 25 µm and in b) and d) to 2.5 µm.

FIG. 12 shows a) JV curves and b) absorption spectrum for typicaldevices consisting of both crosslinked (circles) and annealed (squares)PEDOT:PSS layers. Inset shows the short circuit current density (Jsc,mAcm⁻² ), power conversion efficiency (Eff, %), open circuit voltage(Voc, V) and fill factor (FF) for typical devices of both architectures.

FIG. 13 shows an SEM image of the top view of substrates after thedeposition of a) and c) NiO, and b) and d) V₂O₅ with the formedperovskite layer on top. Scale bars in a) and b) correspond to 25 µm andin c) and d) to 2.5 µm.

FIG. 14 shows JV curves from devices containing a vanadium oxide(squares) and a NiO (circles) p-type contact.

FIG. 15 shows a) temporal evolution of JV curves from the same devicewith illumination time. The scans are performed every minute with thefirst scan lowest down and the last one highest up. b) JV curves forchampion devices of regular (triangles) and our inverted (circles)configuration. Inset shows the short circuit current density (Jsc,mAcm⁻²), power conversion efficiency (Eff, %), open circuit voltage(Voc, V) and fill factor (FF) for both device architectures.

FIG. 16 shows a schematic illustration of a hybrid tandem solar cellarchitecture, here a c-Si HIT (Heterojunction with Intrinsic Thin layer)cell is used as the rear cell in the tandem junction, where i =intrinsic, a = amorphous, c = crystalline, TCO = transparent conductingoxide. Sun light is incident from the top.

FIG. 17 shows a schematic illustration of a hybrid tandem solar cellarchitecture, here a perovskite solar cell is employed as the top cell,with a conventional thin film solar cell as the rear cell in the tandemjunction, where TCO = transparent conducting oxide. Notably for thecurrent generation of thin film technologies (e.g. GIGS solar cells)there is a requirement to realize “inverted” perovskite solar cells formonolithic two terminal tandem devices. Sun light is incident from thetop.

FIG. 18 shows a photograph of an evaporation chamber for dual sourcevapour deposition of a perovskite.

FIG. 19 shows a schematic diagram of a dual source evaporation chamber.

FIG. 20 shows a) completed dual source evaporated perovskite solar cell;b) illustration of the cross-section image; c) cross-section SEM imageof the completed device.

FIG. 21 shows a) sample processed on FTO coated glass with compact TiO₂layer and spin-coated perovskite only; b,c) surface SEM image of thespin-coated perovskite; d) sample with FTO coated glass and compact TiO₂layer and evaporated perovskite only; e,f) surface SEM image of theevaporated perovskite.

FIG. 22 shows J-V curve measured under simulated AM1.5 Sun light of100mWcm⁻² of the best dual source evaporated perovskite solar cell. Theinset gives the solar cell performance parameters derived from this J-Vcurve.

FIG. 23 shows XRD measurement of the evaporated perovskite comparingwith the spin-coated perovskite (termed K330), methylammonium iodide(MAI) lead iodide (PbI₂) and TiO₂ coated FTO glass.

FIG. 24 shows a comparison of the absorbance for 200 nm thick films ofevaporated and spin-coated perovksite.

FIG. 25 shows a comparison between unannealed and annealed layers ofperovskite deposited by two source vapour deposition: left: unannealedevaporated perovskite surface (as evaporated); right: annealedevaporated perovskite surface (after annealing at 100° C. for 45 minutesin a nitrogen glove box).

FIG. 26 shows a comparison of surface coverage by two source vapourdeposition and by solution deposition: left: evaporated and annealedprovskite films; right: spin-coated and annealed perovskite film coatedupon glass/FTO/compact TiO₂.

FIG. 27 shows a comparison between SEM cross-sections: left: two sourceevaporated flat junction device; right: spin-coated flat junctiondevice.

FIG. 28 shows an XRD comparison between a perovskite layer formed by twosource vapour deposition and a perovskite layer formed by spin-coating aperovskite precursor solution. For both films the starting precursorswere MAI and PbCl₂.

FIG. 29 shows cross-section scanning electron micrographs of devicesshowing (from bottom to top) the glass substrate, FTO, TiO₂electron-selective layer, photoactive layer, spiro-OMeTAD. Thephotoactive layers are (a) PbCl₂ and (b) CH₃NH₃PbI_(3-x)CI_(x) after dipcoating a PbCl₂ film in a propan-2-ol solution of CH₃NH₃I.

FIG. 30 shows cross-section scanning electron micrographs of devicesshowing, from bottom to top, the glass substrate, FTO, TiO₂electron-selective layer, photoactive layer, spiro-OMeTAD. Thephotoactive layers are (a) PbI₂ and (b) CH₃NH₃PbI₃ after dip coating aPbI₂ film in a propan-2-ol solution of CH₃NH₃I.

FIG. 31 shows x-ray diffraction spectra of thin-films of (a) PbCl₂, (b)CH₃NH₃PbI_(3-x)Cl_(x,) (c) PbI₂ and (d) CH₃NH₃PbI₃. After dip coating,films from both precursors show a relative intensity decrease of peakscorresponding to the precursor lattice and a relative increase of theperovskite lattice (absent in precursor XRD spectra) indicatingpredominant conversion of the precursor films into perovskite.

FIG. 32 shows current density-voltage characteristics of a device madeusing PbI₂ as the active layer (dashed line) and a device where theevaporated PbI₂ has been converted to CH₃NH₃PbI₃ (solid line) by dipcoating in a methylammonium iodide solution in propan-2-ol. Theperformance parameters for the PbI₂ are J_(sc) = 1.6 mA/cm², PCE =0.80%, V_(oc) = 0.97 V, FF = 0.57. The performance parameters for theCH₃NH₃PbI₃ are J_(sc) = 5.3 mA/cm², PCE = 2.4%, V_(oc) = 0.82 V, FF =0.61.

FIG. 33 shows current density-voltage characteristics of a device madeusing PbCl₂ as the active layer (dashed line) and a device where theevaporated PbCl₂ has been converted to CH₃NH₃PbI_(3-x)Cl_(x) (solidline) by dip coating in a methylammonium iodide solution in propan-2-ol.The performance parameters for the PbCl₂ are J_(sc) = 0.081 mA/cm², PCE= 0.006 %, V_(oc) = 0.29 V, FF = 0.27. The performance parameters forthe CH₃NH₃PbI_(3-x)Cl_(x) are J_(sc) = 19.0 mA/cm², PCE = 7.0%, V_(oc) =0.80 V, FF = 0.49.

FIG. 34 shows photoluminescence measurements and fits to a diffusionmodel for a mixed halide organolead trihalide perovskite filmCH₃NH₃PbI_(3-x)Cl_(x) and a triiodide perovskite film CH₃NH₃PbI₃, in thepresence of p- or n-type quenchers. Time-resolved PL measurements takenat the peak emission wavelength of the mixed halide perovskite with anelectron (PCBM; triangles) or hole (Spiro-OMeTAD; circles) quencherlayer, along with stretched exponential fits to the films coated withinsulating PMMA data (black squares) and fits to the quenching samplesusing the diffusion model described in the text. A pulsed (0.3 to 10MHz) excitation source at 507 nm with a fluence of 30nJ/cm² impinged onthe glass substrate side. Inset in FIG. 34 : Comparison of the PL decayof the two perovskites (with PMMA coating) on a longer time scale, withlifetimes τ_(e) quoted as the time taken to reach ⅟e of the initialintensity.

FIG. 35 shows photoluminescence measurements and fits to a diffusionmodel for an organolead triiodide perovskite film CH₃NH₃PbI, in thepresence of p- or n-type quenchers. Time-resolved PL measurements takenat the peak emission wavelength of the mixed halide perovskite with anelectron (PCBM; triangles) or hole (Spiro-OMeTAD; circles) quencherlayer, along with stretched exponential fits to the films coated withinsulating PMMA data (black squares) and fits to the quenching samplesusing the diffusion model described in the text. A pulsed (0.3 to 10MHz) excitation source at 507 nm with a fluence of 30nJ/cm² impinged onthe glass substrate side.

FIG. 36 shows a cross-sectional SEM image of a 270-nm thick mixed halideabsorber layer with a top hole-quenching layer of Spiro-OMeTAD.

FIG. 37 shows photoluminescence decay for a mixed halide organoleadtrihalide perovskite film CH₃NH₃PbI_(3-x)Cl_(x) (Black squares) and aorganolead triiodide perovskite film CH₃NH₃PbI₃ (grey squares), coatedwith PMMA. lifetimes τ_(e) quoted as the time taken to reach ⅟e of theinitial intensity.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides an optoelectronic device comprising a photoactiveregion. The photoactive region comprises: an n-type region comprising atleast one n-type layer; a p-type region comprising at least one p-typelayer; and, disposed between the n-type region and the p-type region: alayer of a perovskite semiconductor without open porosity.

The term “photoactive region”, as used herein, refers to a region in theoptoelectronic device which (i) absorbs light, which may then generatefree charge carriers; or (ii) accepts charge, both electrons and holes,which may subsequently recombine and emit light.

The term “semiconductor” as used herein refers to a material withelectrical conductivity intermediate in magnitude between that of aconductor and a dielectric. A semiconductor may be an n-typesemiconductor, a p-type semiconductor or an intrinsic semiconductor.

As used herein, the term “n-type region”, refers to a region of one ormore electron-transporting (i.e. n-type) materials. Similarly, the term“n-type layer” refers to a layer of an electron-transporting (i.e. ann-type) material. An electron-transporting (i.e. an n-type) materialcould be a single electron-transporting compound or elemental material,or a mixture of two or more electron-transporting compounds or elementalmaterials. An electron-transporting compound or elemental material maybe undoped or doped with one or more dopant elements.

As used herein, the term “p-type region”, refers to a region of one ormore hole-transporting (i.e. p-type) materials. Similarly, the term“p-type layer” refers to a layer of a hole-transporting (i.e. a p-type)material. A hole-transporting (i.e. a p-type) material could be a singlehole-transporting compound or elemental material, or a mixture of two ormore hole-transporting compounds or elemental materials. Ahole-transporting compound or elemental material may be undoped or dopedwith one or more dopant elements.

The term “perovskite”, as used herein, refers to a material with athree-dimensional crystal structure related to that of CaTiO₃ or amaterial comprising a layer of material, wherein the layer has astructure related to that of CaTiO₃. The structure of CaTiO₃ can berepresented by the formula ABX₃, wherein A and B are cations ofdifferent sizes and X is an anion. In the unit cell, the A cations areat (0,0,0), the B cations are at (½, ½, ½) and the X anions are at (½,½, 0). The A cation is usually larger than the B cation. The skilledperson will appreciate that when A, B and X are varied, the differention sizes may cause the structure of the perovskite material to distortaway from the structure adopted by CaTiO₃ to a lower-symmetry distortedstructure. The symmetry will also be lower if the material comprises alayer that has a structure related to that of CaTiO₃. Materialscomprising a layer of perovskite material are well known. For instance,the structure of materials adopting the K₂NiF₄-type structure comprisesa layer of perovskite material. The skilled person will appreciate thata perovskite material can be represented by the formula [A][B][X]₃,wherein [A] is at least one cation, [B] is at least one cation and [X]is at least one anion. When the perovskite comprise more than one Acation, the different A cations may distributed over the A sites in anordered or disordered way. When the perovskite comprises more than one Bcation, the different B cations may distributed over the B sites in anordered or disordered way. When the perovskite comprise more than one Xanion, the different X anions may distributed over the X sites in anordered or disordered way. The symmetry of a perovskite comprising morethan one A cation, more than one B cation or more than one X cation,will be lower than that of CaTiO₃.

As mentioned in the preceding paragraph, the term “perovskite”, as usedherein, refers to (a) a material with a three-dimensional crystalstructure related to that of CaTiO₃ or (b) a material comprising a layerof material, wherein the layer has a structure related to that ofCaTiO₃. Although both of these categories of perovskite may be used inthe devices according to the invention, it is preferable in somecircumstances to use a perovskite of the first category, (a), i.e. aperovskite having a three-dimensional (3D) crystal structure. Suchperovskites typically comprise a 3D network of perovskite unit cellswithout any separation between layers. Perovskites of the secondcategory, (b), on the other hand, include perovskites having atwo-dimensional (2D) layered structure. Perovskites having a 2D layeredstructure may comprise layers of perovskite unit cells that areseparated by (intercalated) molecules; an example of such a 2D layeredperovskite is [2-(1-cyclohexenyl)ethylammonium]₂PbBr₄. 2D layeredperovskites tend to have high exciton binding energies, which favoursthe generation of bound electron-hole pairs (excitons), rather than freecharge carriers, under photoexcitation. The bound electron-hole pairsmay not be sufficiently mobile to reach the p-type or n-type contactwhere they can then transfer (ionise) and generate free charge.Consequently, in order to generate free charge, the exciton bindingenergy has to be overcome, which represents an energetic cost to thecharge generation process and results in a lower voltage in aphotovoltaic cell and a lower efficiency. In contrast, perovskiteshaving a 3D crystal structure tend to have much lower exciton bindingenergies (on the order of thermal energy) and can therefore generatefree carriers directly following photoexcitation. Accordingly, theperovskite semiconductor employed in the devices and processes of theinvention is preferably a perovskite of the first category, (a), i.e. aperovskite which has a three-dimensional crystal structure. This isparticularly preferable when the optoelectronic device is a photovoltaicdevice.

The perovskite semiconductor employed in the present invention, in saidlayer of a perovskite semiconductor without open porosity, is typicallyone which is capable of (i) absorbing light, and thereby generating freecharge carriers; and/or (ii) emitting light, by accepting charge, bothelectrons and holes, which subsequently recombine and emit light. Thus,the perovskite employed is typically a light-absorbing and/or alight-emitting perovskite.

As the skilled person will appreciate, the perovskite semiconductoremployed in the present invention, in said layer of a perovskitesemiconductor without open porosity, may be a perovskite which acts asan n-type, electron-transporting semiconductor when photo-doped.Alternatively, it may be a perovskite which acts as a p-typehole-transporting semiconductor when photo-doped. Thus, the perovskitemay be n-type or p-type, or it may be an intrinsic semiconductor. Inpreferred embodiments, the perovskite employed is one which acts as ann-type, electron-transporting semiconductor when photo-doped.

Typically, the perovskite semiconductor used in the present invention isa photosensitizing material, i.e. a material which is capable ofperforming both photogeneration and charge (electron or hole)transportation.

As used herein, the term “porous” refers to a material within whichpores are arranged. Thus, for instance, in a porous scaffold materialthe pores are volumes within the scaffold where there is no scaffoldmaterial. The individual pores may be the same size or different sizes.The size of the pores is defined as the “pore size”. The limiting sizeof a pore, for most phenomena in which porous solids are involved, isthat of its smallest dimension which, in the absence of any furtherprecision, is referred to as the width of the pore (i.e. the width of aslit-shaped pore, the diameter of a cylindrical or spherical pore,etc.). To avoid a misleading change in scale when comparing cylindricaland slit-shaped pores, one should use the diameter of a cylindrical pore(rather than its radius) as its “pore-width “ (J. Rouquerol et al.,“Recommendations for the Characterization of Porous Solids”, Pure &Appl. Chem., Vol. 66, No. 8, pp. 1739-1758, 1994). The followingdistinctions and definitions were adopted in previous IUPAC documents(K.S.W. Sing, et al, Pure and Appl. Chem., vol.57, n04, pp 603-919,1985; and IUPAC “Manual on Catalyst Characterization”, J. Haber, Pureand Appl. Chem., vo1.63, pp. 1227-1246, 1991):

-   Micropores have widths (i.e. pore sizes) smaller than 2 nm.-   Mesopores have widths (i.e. pore sizes) of from 2 nm to 50 nm.-   –- Macropores have widths (i.e. pore sizes) of greater than 50 nm.

Pores in a material may include “closed” pores as well as open pores. Aclosed pore is a pore in a material which is a non-connected cavity,i.e. a pore which is isolated within the material and not connected toany other pore and which cannot therefore be accessed by a fluid (e.g. aliquid, such as a solution) to which the material is exposed. An “openpore” on the other hand, would be accessible by such a fluid. Theconcepts of open and closed porosity are discussed in detail in J.Rouquerol et al., “Recommendations for the Characterization of PorousSolids”, Pure & Appl. Chem., Vol. 66, No. 8, pp. 1739-1758, 1994.

Open porosity, therefore, refers to the fraction of the total volume ofthe porous material in which fluid flow could effectively take place. Ittherefore excludes closed pores. The term “open porosity” isinterchangeable with the terms “connected porosity” and “effectiveporosity”, and in the art is commonly reduced simply to “porosity”. (Theperovskite semiconductor present in the “layer of a perovskitesemiconductor without open porosity”, in the optoelectronic device ofthe invention, cannot therefore be said to be a “porous perovskite”.)

The term “without open porosity”, as used herein, therefore refers to amaterial with no effective porosity.

The optoelectronic device of the present invention comprises a layer ofa perovskite semiconductor without open porosity. That layer, and theperovskite semiconductor within it, are without open porosity. Theperovskite semiconductor in the layer is not therefore infiltrated bythe, or any of the, n-type material(s) in the n-type region, andlikewise it is not infiltrated by the, or any of the, p-type material(s)in the p-type region. Rather, the perovskite semiconductor in that layertypically forms a planar heterojunction with the n-type or the p-typeregion, or in some cases it forms planar heterojunctions with both then-type region and the p-type region.

Also, when the layer of the perovskite semiconductor without openporosity is a “capping layer”, which is disposed on a first layer whichcomprises a scaffold material and a perovskite semiconductor, thecapping layer is not infiltrated by the scaffold material either,because the capping layer and the perovskite semiconductor within thecapping layer are without open porosity. The perovskite in the firstlayer, on the other hand (which is generally the same perovskitecompound as the perovskite compound in the capping layer), is typicallydisposed in pores of the scaffold material and may therefore be said tobe “infiltrated” by the scaffold material.

In some embodiments of the optoelectronic device of the presentinvention, the layer of the perovskite semiconductor without openporosity is non-porous. The term “non-porous” as used herein, refers toa material without any porosity, i.e. without open porosity and alsowithout closed porosity.

Generally, the layer of the perovskite semiconductor without openporosity consists essentially of the perovskite semiconductor. Aperovskite is a crystalline compound. Thus, the layer of the perovskitesemiconductor without open porosity typically consists essentially ofcrystallites of the perovskite. In some embodiments, the layer of theperovskite semiconductor without open porosity consists of theperovskite semiconductor. Thus, typically the layer of the perovskitesemiconductor without open porosity consists of crystallites of theperovskite.

The layer of the perovskite semiconductor without open porosity isgenerally in contact with at least one of the n-type region or thep-type region.

The layer of the perovskite semiconductor without open porositytypically forms a planar heterojunction with the n-type region or thep-type region. Either the n-type region or the p-type region may bedisposed on the layer of the perovskite semiconductor without openporosity, but as explained above, since the layer of the perovskitesemiconductor is without open porosity the n-type or p-type materialdoes not infiltrate the perovskite semiconductor to form a bulkheterojunction; rather it usually forms a planar heterojunction with theperovskite semiconductor. Typically, the layer of the perovskitesemiconductor without open porosity forms a planar heterojunction withthe n-type region.

In some embodiments, the layer of the perovskite semiconductor withoutopen porosity is in contact with both the n-type region and the p-typeregion. In such embodiments there will be no other layer (such as a“first layer” which comprises a scaffold material and a perovskitesemiconductor) separating the layer of the perovskite semiconductorwithout open porosity from the n-type region or the p-type region. Asexplained above, since the layer of the perovskite semiconductor iswithout open porosity, in such embodiments neither the n-type region orthe p-type region material infiltrates the perovskite semiconductor toform a bulk heterojunction; rather it usually forms a planarheterojunction with the perovskite semiconductor. Thus, the layer of theperovskite semiconductor without open porosity may form planarheterojunctions with both the n-type and p-type regions on either sideof the layer. Accordingly, in some embodiments of the optoelectronicdevice of the invention, the layer of the perovskite semiconductor formsa first planar heterojunction with the n-type region and a second planarheterojunction with the p-type region.

The optoelectronic device of the invention is typically a thin filmdevice.

Usually, the thickness of the layer of the perovskite semiconductorwithout open porosity is from 10 nm to 100 µm. More typically, thethickness of the layer of the perovskite semiconductor without openporosity is from 10 nm to 10 µm. Preferably, the thickness of the layerof the perovskite semiconductor without open porosity is from 50 nm to1000 nm, for instance from 100 nm to 700 nm. The thickness of the layerof the perovskite semiconductor is often greater than 100 nm. Thethickness may for example be from 100 nm to 100 µm, or for instance from100 nm to 700 nm.

In order to provide highly efficient photovoltaic devices, theabsorption of the absorber / photoactive region should ideally bemaximised so as to generate an optimal amount of current. Consequently,when using a perovskite as the absorber in a solar cell, the thicknessof the perovskite layer should ideally be in the order of from 300 to600 nm, in order to absorb most of the sun light across the visiblespectrum. In particular, in a solar cell the perovskite layer shouldgenerally be thicker than the absorption depth (which is defined as thethickness of film required to absorb 90% of the incident light of agiven wavelength, which for the perovskite materials of interest istypically above 100 nm if significant light absorption is requiredacross the whole visible spectrum (400 to 800 nm)), as the use of aphotoactive layer in photovoltaic devices with a thickness of less than100 nm can be detrimental to the performance of the device.

In contrast, electroluminescent (light-emitting) devices do not need toabsorb light and are therefore not constrained by the absorption depth.Moreover, in practice the p-type and n-type contacts ofelectroluminescent devices are typically chosen such that once anelectron or hole is injected on one side of the device it will not flowout of the other (i.e. they are selected so as to only inject or collecta single carrier), irrespective of the thickness of the photoactivelayer. In essence, the charge carriers are blocked from transferring outof the photoactive region and will thereby be available to recombine andgenerate photons, and can therefore make use of a photoactive regionthat is significantly thinner.

Typically, therefore, when the optoelectronic device is a photovoltaicdevice, the thickness of the layer of the perovskite semiconductor isgreater than 100 nm. The thickness of the layer of the perovskitesemiconductor in the photovoltaic device may for instance be from 100 nmto 100 µm, or for instance from 100 nm to 700 nm. The thickness of thelayer of the perovskite semiconductor in the photovoltaic device may forinstance be from 200 nm to 100 µm, or for instance from 200 nm to 700nm.

As used herein, the term “thickness” refers to the average thickness ofa component of an optoelectronic device.

The inventors have shown that a thin scaffold may be used to seed thegrowth of the photoactive perovskite layer where the majority of thephotoactivity (e.g. light absorption) occurs in a capping layer thatforms above the scaffold. This capping layer is the abovementioned layerof the perovskite semiconductor without open porosity, and, in theseembodiments, a “first layer” separates that capping layer from eitherthe n-type region or the p-type region.

Accordingly, in some embodiments, said photoactive region of the devicecomprises:

-   said n-type region;-   said p-type region; and, disposed between the n-type region and the    p-type region:    -   (i) a first layer which comprises a scaffold material and a        perovskite semiconductor; and    -   (ii) a capping layer disposed on said first layer, which capping        layer is said layer of a perovskite semiconductor without open        porosity.

The perovskite semiconductor in the capping layer is in contact with theperovskite semiconductor in the first layer.

Since the perovskite in the first layer and the perovskite in thecapping layer are often deposited together in the same step, typicallyby the same solution deposition or vapour deposition step, theperovskite semiconductor in the capping layer is usually made of thesame perovskite compound as the perovskite semiconductor in the firstlayer.

Unlike the first layer, which comprises both the scaffold material andthe perovskite semiconductor, the capping layer does not comprise thescaffold material. As explained above, the capping layer, which is saidlayer of a perovskite semiconductor without open porosity, typicallyconsists essentially of, or consists of crystallites of the perovskitesemiconductor. The capping layer usually therefore consists essentiallyof the perovskite semiconductor. In some embodiments the capping layerconsists of the perovskite semiconductor.

The first layer comprises said scaffold material and said perovskitesemiconductor disposed on the surface of the scaffold material. The term“scaffold material” as used herein refers to a material whosefunction(s) include acting as a physical support for another material.In the present case, the scaffold material acts as a support for theperovskite semiconductor present in the first layer. The perovskitesemiconductor is disposed, or supported on, the surface of the scaffoldmaterial. The scaffold material is usually porous, meaning that ittypically has an open porous structure. Accordingly, the “surface” ofthe scaffold material here typically refers to the surfaces of poreswithin the scaffold material. Thus, the perovskite semiconductor in thefirst layer is typically disposed on the surfaces of pores within thescaffold material.

In some embodiments, the scaffold material is porous and the perovskitesemiconductor in the first layer is disposed in pores of the scaffoldmaterial. The effective porosity of said scaffold material is usually atleast 50%. For instance, the effective porosity may be about 70%. In oneembodiment, the effective porosity is at least 60%, for instance atleast 70%.

The scaffold material is usually mesoporous. The term “mesoporous”, asused herein, means that the mean pore size of the pores within thematerial is from 2 nm to 50 nm. Individual pores may be different sizesand may be any shape.

Alternatively, the scaffold material may be macroporous. The term“macroporous”, as used herein, means that the mean pore size of thepores within the material is greater than 2 nm. In some embodiments, thepore size in the scaffold material, when it is macroporous, is greaterthan 2 nm and equal to or less than 1 µm, or for instance, greater than2 nm and equal to or less than 500 nm, more preferably greater than 2 nmand equal to or less than 200 nm.

The scaffold material may be a charge-transporting scaffold material(e.g. an electron-transporting material such as titania, oralternatively a hole transporting material) or a dielectric material,such as alumina. The term “dielectric material”, as used herein, refersto material which is an electrical insulator or a very poor conductor ofelectric current. The term dielectric therefore excludes semiconductingmaterials such as titania. The term dielectric, as used herein,typically refers to materials having a band gap of equal to or greaterthan 4.0 eV. (The band gap of titania is about 3.2 eV.) The skilledperson of course is readily able to measure the band gap of asemiconductor, by using well-known procedures which do not require undueexperimentation. For instance, the band gap of the semiconductor can beestimated by constructing a photovoltaic diode or solar cell from thesemiconductor and determining the photovoltaic action spectrum. Themonochromatic photon energy at which the photocurrent starts to begenerated by the diode can be taken as the band gap of thesemiconductor; such a method was used by Barkhouse et al., Prog.Photovolt: Res. Appl. 2012; 20:6-11. References herein to the band gapof the semiconductor mean the band gap as measured by this method, i.e.the band gap as determined by recording the photovoltaic action spectrumof a photovoltaic diode or solar cell constructed from the semiconductorand observing the monochromatic photon energy at which significantphotocurrent starts to be generated.

Usually, the perovskite semiconductor in the first layer (which layeralso comprises the scaffold material) contacts one of the p-type andn-type regions, and the perovskite semiconductor in the capping layercontacts the other of the p-type and n-type regions. Typically, theperovskite semiconductor in the capping layer forms a planarheterojunction with the region with which it is in contact, i.e. withthe p-type region or the n-type region.

In one preferred embodiment, the perovskite semiconductor in the cappinglayer contacts the p-type region, and the perovskite semiconductor inthe first layer contacts the n-type region. Usually, in this embodiment,the scaffold material is either an electron-transporting scaffoldmaterial or a dielectric scaffold material. Typically, the perovskitesemiconductor in the capping layer forms a planar heterojunction withthe p-type region.

In another embodiment, however, the perovskite semiconductor in thecapping layer contacts the n-type region, and the perovskitesemiconductor in the first layer contacts the p-type region. Typically,in this embodiment, the scaffold material is a hole-transportingscaffold material or a dielectric scaffold material. Typically, theperovskite semiconductor in the capping layer forms a planarheterojunction with the n-type region.

The thickness of the capping layer is usually greater than the thicknessof the first layer. The majority of the photoactivity (e.g. lightabsorption) therefore usually occurs in a capping layer.

The thickness of the capping layer is typically from 10 nm to 100 µm.More typically, the thickness of the capping layer is from 10 nm to 10µm. Preferably, the thickness of the capping layer is from 50 nm to 1000nm, or for instance from 100 nm to 700 nm.

The thickness of the capping layer may for example be from 100 nm to 100µm, or for instance from 100 nm to 700 nm. A capping layer having athickness of at least 100 nm is usually preferred.

The thickness of the first layer, on the other hand, is often from 5 nmto 1000 nm. More typically, it is from 5 nm to 500 nm, or for instancefrom 30 nm to 200 nm.

The perovskite semiconductor employed in the present invention, in saidlayer of a perovskite semiconductor without open porosity, and, whenpresent, in said first layer, is typically one which is capable of (i)absorbing light, and thereby generating free charge carriers; and/or(ii) emitting light, by accepting charge, both electrons and holes,which subsequently recombine and emit light.

Thus, the perovskite employed is typically a light-absorbing and/or alight-emitting perovskite.

Usually, the perovskite is a light-absorbing material. Typically aperovskite is employed which is capable of absorbing light having awavelength of from 300 to 2000 nm (i.e. a perovskite which is capable ofabsorbing light that has a wavelength which falls anywhere within thisrange). More typically, the perovskite employed is one which is capableof absorbing light having a wavelength in the range of from 300 to 1200nm, or, for instance, capable of absorbing light having a wavelength offrom 300 to 1000 nm. More typically, the perovskite employed is onewhich is capable of absorbing light having a wavelength anywhere in therange of from 300 to 800 nm.

The perovskite semiconductor employed in the optoelectronic device ofthe invention preferably has a band gap which is narrow enough to allowthe excitation of electrons by incident light. A band gap of 3.0 eV orless is particularly preferred, especially when the optoelectronicdevice is a photovoltaic device, because such a band gap is low enoughfor sunlight to excite electrons across it. Certain perovskites,including some oxide perovskites and 2D layered perovskites, have bandgaps that are wider than 3.0 eV, and are therefore less preferred foruse in photovoltaic devices than perovskites which have a band gap of3.0 eV or less. Such perovskites include CaTiO₃, SrTiO₃ andCaSrTiO₃:Pr³⁺, which have band gaps of around 3.7 eV, 3.5 eV and 3.5 eVrespectively.

Accordingly, the perovskite semiconductor employed in the optoelectronicdevice of the invention typically has a band gap of equal to or lessthan 3.0 eV. In some embodiments, the band gap of the perovskite is lessthan or equal to 2.8 eV, for instance equal to or less than 2.5 eV. Theband gap may for instance be less than or equal to 2.3 eV, or forinstance less than or equal to 2.0 eV.

Usually, the band gap is at least 0.5 eV. Thus, the band gap of theperovskite may be from 0.5 eV to 2.8 eV. In some embodiments it is from0.5 eV to 2.5 eV, or for example from 0.5 eV to 2.3 eV. The band gap ofthe perovskite may for instance be from 0.5 eV to 2.0 eV. In otherembodiments, the band gap of the perovskite may be from 1.0 eV to 3.0eV, or for instance from 1.0 eV to2.8 eV. In some embodiments it is from1.0 eV to 2.5 eV, or for example from 1.0 eV to 2.3 eV. The band gap ofthe perovskite semiconductor may for instance be from 1.0 eV to 2.0 eV.

The band gap of the perovskite is more typically from 1.2 eV to 1.8 eV.The band gaps of organometal halide perovskite semiconductors, forexample, are typically in this range and may for instance, be about 1.5eV or about 1.6 eV. Thus, in one embodiment the band gap of theperovskite is from 1.3 eV to 1.7 eV.

The perovskite semiconductor employed in the optoelectronic device ofthe invention typically comprises at least one anion selected fromhalide anions and chalcogenide anions.

The term “halide” refers to an anion of a group 7 element, i.e., of ahalogen. Typically, halide refers to a fluoride anion, a chloride anion,a bromide anion, an iodide anion or an astatide anion. The term“chalcogenide anion”, as used herein refers to an anion of a group 6element, i.e. of a chalcogen. Typically, chalcogenide refers to an oxideanion, a sulphide anion, a selenide anion or a telluride anion.

In the optoelectronic device of the invention, the perovskite oftencomprises a first cation, a second cation, and said at least one anion.

As the skilled person will appreciate, the perovskite may comprisefurther cations or further anions. For instance, the perovskite maycomprise two, three or four different first cations; two, three or fourdifferent second cations; or two, three of four different anions.

Typically, in the optoelectronic device of the invention, the secondcation in the perovskite is a metal cation. The metal may be selectedfrom tin, lead and copper, and is preferably selected from tin and lead.

More typically, the second cation is a divalent metal cation. Forinstance, the second cation may be selected from Ca²⁺, Sr²⁺, Cd²⁺, Cu²⁺,Ni2⁺, Mn2⁺, Fe²⁺, Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺, Sn²⁺, Yb²⁺ and Eu²⁺ .The second cation may be selected from Sn²⁺, Pb²⁺ and Cu²⁺. Usually, thesecond cation is selected from Sn²⁺ and Pb²⁺.

In the optoelectronic device of the invention, the first cation in theperovskite is usually an organic cation.

The term “organic cation” refers to a cation comprising carbon. Thecation may comprise further elements, for example, the cation maycomprise hydrogen, nitrogen or oxygen.

Usually, in the optoelectronic device of the invention, the organiccation has the formula (R₁R₂R₃R₄N)⁺, wherein:

-   R₁ is hydrogen, unsubstituted or substituted C₁-C₂₀ alkyl, or    unsubstituted or substituted aryl;-   R₂ is hydrogen, unsubstituted or substituted C₁-C₂₀ alkyl, or    unsubstituted or substituted aryl;-   R₃ is hydrogen, unsubstituted or substituted C₁-C₂₀ alkyl, or    unsubstituted or substituted aryl; and-   R₄ is hydrogen, unsubstituted or substituted C₁-C₂₀ alkyl, or    unsubstituted or substituted aryl.

Alternatively, the organic cation may have the formula (R₅NH₃)⁺,wherein: R₅ is hydrogen, or unsubstituted or substituted C₁-C₂₀ alkyl.For instance, R₅ may be methyl or ethyl. Typically, R₅ is methyl.

In some embodiments, the organic cation has the formula(R₅R₆N═CH—NR₇R₈)⁺, wherein: R₅ is hydrogen, unsubstituted or substitutedC₁-C₂₀ alkyl, or unsubstituted or substituted aryl; R₆ is hydrogen,unsubstituted or substituted C₁-C₂₀ alkyl, or unsubstituted orsubstituted aryl; R₇ is hydrogen, unsubstituted or substituted C₁-C₂₀alkyl, or unsubstituted or substituted aryl; and R₈ is hydrogen,unsubstituted or substituted C₁-C₂₀ alkyl, or unsubstituted orsubstituted aryl.

Typically, R₅ in the cation (R₅R₆N═CH—NR₇R₈)⁺ is hydrogen, methyl orethyl, R₆ is hydrogen, methyl or ethyl, R₇ is hydrogen, methyl or ethyl,and R₈ is hydrogen, methyl or ethyl. For instance R₅ may be hydrogen ormethyl, R₆ may be hydrogen or methyl, R₇ may be hydrogen or methyl, andR₈ may be hydrogen or methyl.

The organic cation may, for example, have the formula (H₂N═CH—NH₂)⁺.

As used herein, an alkyl group can be a substituted or unsubstituted,linear or branched chain saturated radical, it is often a substituted oran unsubstituted linear chain saturated radical, more often anunsubstituted linear chain saturated radical. A C₁-C₂₀ alkyl group is anunsubstituted or substituted, straight or branched chain saturatedhydrocarbon radical. Typically it is C₁-C₁₀ alkyl, for example methyl,ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl or decyl, orC₁-C₆ alkyl, for example methyl, ethyl, propyl, butyl, pentyl or hexyl,or C₁-C₄ alkyl, for example methyl, ethyl, i-propyl, n-propyl, t-butyl,s-butyl or n-butyl.

When an alkyl group is substituted it typically bears one or moresubstituents selected from substituted or unsubstituted C₁-C₂₀ alkyl,substituted or unsubstituted aryl (as defined herein), cyano, amino,C₁-C₁₀ alkylamino, di(C₁-C₁₀)alkylamino, arylamino, diarylamino,arylalkylamino, amido, acylamido, hydroxy, oxo, halo, carboxy, ester,acyl, acyloxy, C₁-C₂₀ alkoxy, aryloxy, haloalkyl, sulfonic acid,sulfhydryl (i.e. thiol, -SH), C₁-C₁₀ alkylthio, arylthio, sulfonyl,phosphoric acid, phosphate ester, phosphonic acid and phosphonate ester.Examples of substituted alkyl groups include haloalkyl, hydroxyalkyl,aminoalkyl, alkoxyalkyl and alkaryl groups. The term alkaryl, as usedherein, pertains to a C₁-C₂₀ alkyl group in which at least one hydrogenatom has been replaced with an aryl group. Examples of such groupsinclude, but are not limited to, benzyl (phenylmethyl, PhCH2-),benzhydryl (Ph₂CH-), trityl (triphenylmethyl, Ph₃C-), phenethyl(phenylethyl, Ph—CH₂CH₂—), styryl (Ph—CH═CH—), cinnamyl (Ph—CH═CH—CH₂—).

Typically a substituted alkyl group carries 1, 2 or 3 substituents, forinstance 1 or 2.

An aryl group is a substituted or unsubstituted, monocyclic or bicyclicaromatic group which typically contains from 6 to 14 carbon atoms,preferably from 6 to 10 carbon atoms in the ring portion. Examplesinclude phenyl, naphthyl, indenyl and indanyl groups. An aryl group isunsubstituted or substituted. When an aryl group as defined above issubstituted it typically bears one or more substituents selected fromC₁-C₆ alkyl which is unsubstituted (to form an aralkyl group), arylwhich is unsubstituted, cyano, amino, C₁-C₁₀ alkylamino,di(C₁-C₁₀)alkylamino, arylamino, diarylamino, arylalkylamino, amido,acylamido, hydroxy, halo, carboxy, ester, acyl, acyloxy, C₁-C₂₀ alkoxy,aryloxy, haloalkyl, sulfhydryl (i.e. thiol, -SH), C₁₋₁₀ alkylthio,arylthio, sulfonic acid, phosphoric acid, phosphate ester, phosphonicacid and phosphonate ester and sulfonyl. Typically it carries 0, 1, 2 or3 substituents. A substituted aryl group may be substituted in twopositions with a single C₁-C₆ alkylene group, or with a bidentate grouprepresented by the formula -X-(C₁-C₆)alkylene, or —X—(C₁-C₆)alkylene—X—,wherein X is selected from O, S and NR, and wherein R is H, aryl orC₁-C₆ alkyl. Thus a substituted aryl group may be an aryl group fusedwith a cycloalkyl group or with a heterocyclyl group. The ring atoms ofan aryl group may include one or more heteroatoms (as in a heteroarylgroup). Such an aryl group (a heteroaryl group) is a substituted orunsubstituted mono- or bicyclic heteroaromatic group which typicallycontains from 6 to 10 atoms in the ring portion including one or moreheteroatoms. It is generally a 5- or 6-membered ring, containing atleast one heteroatom selected from O, S, N, P, Se and Si. It maycontain, for example, 1, 2 or 3 heteroatoms. Examples of heteroarylgroups include pyridyl, pyrazinyl, pyrimidinyl, pyridazinyl, furanyl,thienyl, pyrazolidinyl, pyrrolyl, oxazolyl, oxadiazolyl, isoxazolyl,thiadiazolyl, thiazolyl, isothiazolyl, imidazolyl, pyrazolyl, quinolyland isoquinolyl. A heteroaryl group may be unsubstituted or substituted,for instance, as specified above for aryl. Typically it carries 0, 1, 2or 3 substituents.

Mainly, in the optoelectronic device of the invention, R₁ in the organiccation is hydrogen, methyl or ethyl, R₂ is hydrogen, methyl or ethyl, R₃is hydrogen, methyl or ethyl, and R₄ is hydrogen, methyl or ethyl. Forinstance R₁ may be hydrogen or methyl, R₂ may be hydrogen or methyl, R₃may be hydrogen or methyl, and R₄ may be hydrogen or methyl.

Alternatively, the organic cation may have the formula (R₅NH3)⁺,wherein: R₅ is hydrogen, or unsubstituted or substituted C₁-C₂₀ alkyl.For instance, R₅ may be methyl or ethyl. Typically, R₅ is methyl.

In one embodiment, the perovskite is a mixed-anion perovskite comprisingtwo or more different anions selected from halide anions andchalcogenide anions. Usually, said two or more different anions are twoor more different halide anions.

Thus, the perovskite employed may be a mixed-anion perovskite comprisinga first cation, a second cation, and two or more different anionsselected from halide anions and chalcogenide anions. For instance, themixed-anion perovskite may comprise two different anions and, forinstance, the anions may be a halide anion and a chalcogenide anion, twodifferent halide anions or two different chalcogenide anions. The firstand second cations may be as further defined hereinbefore. Thus thefirst cation may be an organic cation, which may be as further definedherein. For instance it may be a cation of formula (R₁R₂R₃R₄N) , orformula (R₅NH₃)⁺, as defined above. Alternatively, the organic cationmay be a cation of formula [R₅R₆N═CH—NR₇R₈]⁺ as defined above. Thesecond cation may be a divalent metal cation. For instance, the secondcation may be selected from Ca²⁺, Sr²⁺, Cd²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Fe²⁺,Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ and Eu²⁺. Usually, the second cationis selected from Sn²⁺ and Pb²⁺.

In the optoelectronic device of the invention, the perovskite is usuallya mixed-halide perovskite, wherein said two or more different anions aretwo or more different halide anions. Typically, they are two or threehalide anions, more typically, two different halide anions. Usually thehalide anions are selected from fluoride, chloride, bromide and iodide,for instance chloride, bromide and iodide.

Often, in the optoelectronic device of the invention, the perovskite isa perovskite compound of the formula (I):

wherein:

-   [A] is at least one organic cation;-   [B] is at least one metal cation; and-   [X] is said at least one anion.

The perovskite of formula (I) may comprise one, two, three or fourdifferent metal cations, typically one or two different metal cations.Also, the perovskite of the formula (I), may, for instance, compriseone, two, three or four different organic cations, typically one or twodifferent organic cations. Likewise, the perovskite of formula (I), may,comprise one two, three or four different anions, typically two or threedifferent anions.

The organic and metal cations in the perovskite compound of formula (I)may be as further defined hereinbefore. Thus the organic cations may beselected from cations of formula (R₁R₂R₃R₄N)⁺ and cations of formula(R₅NH₃)⁺, as defined above. The metal cations may be selected fromdivalent metal cations. For instance, the metal cations may be selectedfrom Ca²⁺, Sr²⁺, Cd²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Fe²⁺, Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺,Pb²⁺, Yb²⁺ and Eu²⁺. Usually, the metal cation is Sn²⁺ or Pb²⁺.

The organic cation may, for instance, be selected from cations offormula (R₅R₆N═CH—NR₇R₈)⁺, and cations of formula (H₂N═CH—NH₂)⁺, asdefined above. The metal cations may be selected from divalent metalcations. For instance, the metal cations may be selected from Ca²⁺, Sr²⁺, Cd²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Fe²⁺, Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ andEu²⁺. Usually, the metal cation is Sn²⁺ or Pb²⁺.

Typically, [X] in formula (I) is two or more different anions selectedfrom halide anions and chalcogenide anions. More typically, [X] is twoor more different halide anions.

In one embodiment, the perovskite is a perovskite compound of theformula (IA):

wherein:

-   A is an organic cation;-   B is a metal cation; and-   [X] is two or more different halide anions.

Typically, [X] in formula (IA) is two or more different anions selectedfrom halide anions and chalcogenide anions. Usually, [X] is two or moredifferent halide anions. Preferably, [X] is two or three differenthalide anions. More preferably, [X] is two different halide anions. Inanother embodiment [X] is three different halide anions.

The organic and metal cations in the perovskite compound of formula (IA)may be as further defined hereinbefore. Thus the organic cation may beselected from cations of formula (R₁R₂R₃R₄N)⁺ and cations of formula(R₅NH₃)⁺, as defined above. The metal cation may be a divalent metalcation. For instance, the metal cation may be selected from Ca²⁺, Sr²⁺,Cd²⁺, Cu²⁺, Ni²⁺, Mn2⁺, Fe²⁺, Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ andEu²⁺. Usually, the metal cation is Sn²⁺ or Pb²⁺.

The organic cation may, for instance, be selected from cations offormula (R₅R₆N═CH—NR₇R₈)⁺, and cations of formula (H₂N═CH—NH₂)⁺, asdefined above. The metal cation may be a divalent metal cation. Forinstance, the metal cation may be selected from Ca²⁺, Sr²⁺, Cd²⁺, Cu²⁺,Ni²⁺, Mn2⁺, Fe²⁺, Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ and Eu²⁺. Usually,the metal cation is Sn²⁺ or Pb²⁺.

Typically, in the optoelectronic device of the invention, the perovskiteis a perovskite compound of formula (II):

wherein:

-   A is an organic cation;-   B is a metal cation;-   X is a first halide anion;-   X′ is a second halide anion which is different from the first halide    anion; and-   y is from 0.05 to 2.95.

Usually, y is from 0.5 to 2.5, for instance from 0.75 to 2.25.Typically, y is from 1 to 2.

Again, in formula (II), the organic and metal cations may be as furtherdefined hereinbefore. Thus the organic cation may be a cation of formula(R₁R₅R₃R₄N)⁺ or, more typically, a cation of formula (R₅NH₃)⁺, asdefined above. The metal cation may be a divalent metal cation. Forinstance, the metal cation may be selected from Ca²⁺, Sr²⁺, Cd²⁺, Cu²⁺,Ni²⁺, Mn²⁺, Fe²⁺, Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ and Eu²⁺. Usually,the metal cation is Sn²⁺ or Pb²⁺.

In some embodiments, the perovskite is a perovskite compound of formula(IIa):

wherein:

-   A is an organic cation of the formula (R₅R₆N═CH—NR₇R₈)⁺, wherein: R₅    is hydrogen, unsubstituted or substituted C₁-C₂₀ alkyl, or    unsubstituted or substituted aryl; R₆ is hydrogen, unsubstituted or    substituted C₁-C₂₀ alkyl, or unsubstituted or substituted aryl; R₇    is hydrogen, unsubstituted or substituted C₁-C₂₀ alkyl, or    unsubstituted or substituted aryl; and R₈ is hydrogen, unsubstituted    or substituted C₁-C₂₀ alkyl, or unsubstituted or substituted aryl;-   B is a metal cation;-   X is a first halide anion;-   X′ is a second halide anion which is different from the first halide    anion; and-   z is greater than 0 and less than 1.

Usually, z is from 0.05 to 0.95.

Usually, z is from 0.1 to 0.9. z may, for instance, be 0.1, 0.2, 0.3,0.4, 0.5, 0.6, 0.7, 0.8 or 0.9, or z may be a range of from any one ofthese values to any other of these values (for instance, from 0.2 to0.7, or from 0.1 to 0.8).

B, X and X′ may be as defined hereinbefore. The organic cation may, forinstance, be (R₅R₆N═CH—NR₇R₈)⁺, wherein: R₅, R₆, R₇ and R₈ areindependently selected from hydrogen and unsubstituted or substitutedC₁-C₆ alkyl. For instance, the organic cation may be (H₂N═CH—NH₂)+.

Often, in the optoelectronic device of the invention, the perovskite isa perovskite compound selected from CH₃NH₃PbI₃, CH₃NH₃PbBr₃,CH₃NH₃PbCl₃, CH₃NH₃PbF₃, CH₃NH₃PbBrI₂, CH₃NH₃PbBrCl₂, CH₃NH₃PbIBr₂,CH₃NH₃PbICl₂, CH₃NH₃PbClBr₂, CH₃NH₃PbI₂Cl, CH₃NH₃SnBrI₂, CH₃NH₃SnBrCl₂,CH₃NH₃SnF₂Br, CH₃NH₃SnIBr₂, CH₃NH₃SnICl₂, CH₃NH₃SnF₂I, CH₃NH₃SnClBr₂,CH₃NH₃SnI₂Cl and CH₃NH₃SnF₂Cl.

For instance, in the optoelectronic device of the invention, theperovskites may be selected from CH₃NH₃PbBrI₂, CH₃NH₃PbBrCl₂,CH₃NH₃PbIBr₂, CH₃NH₃PbICl₂, CH₃NH₃PbClBr₂, CH₃NH₃PbI₂Cl, CH₃NH₃SnBrI₂,CH₃NH₃SnBrCl₂, CH₃NH₃SnF₂Br, CH₃NH₃SnIBr₂, CH₃NH₃SnICl₂, CH₃NH₃SnF₂I,CH₃NH₃SnClBr₂, CH₃NH₃SnI₂Cl and CH₃NH₃SnF₂Cl.

Typically, the perovskite is selected from CH₃NH₃PbBrI₂, CH₃NH₃PbBrCl₂,CH₃NH₃PbIBr₂, CH₃NH₃PbICl₂, CH₃NH₃PbClBr₂, CH₃NH₃PbI₂Cl, CH₃NH₃SnF₂Br,CH₃NH₃SnICl₂, CH₃NH₃SnF₂I, CH₃NH₃SnI₂Cl and CH₃NH₃SnF₂Cl.

More typically, the perovskite is selected from CH₃NH₃PbBrI₂,CH₃NH₃PbBrCl₂, CH₃NH₃PbIBr₂, CH₃NH₃PbICl₂, CH₃NH₃PbClBr₂, CH₃NH₃PbI₂Cl,CH₃NH₃SnF₂Br, CH₃NH₃SnF₂I and CH₃NH₃SnF₂Cl.

Usually, the perovskite is selected from CH₃NH₃PbBrI₂, CH₃NH₃PbBrCl₂,CH₃NH₃PbIBr₂, CH₃NH₃PbICl₂, CH₃NH₃SnF₂Br, and CH₃NH₃SnF₂I.

Often, the perovskite employed is CH₃NH₃PbCl₂I.

In some embodiments, the perovskite may be a perovskite of formula(H₂N=CH-NH₂)PbI_(3z)Br_(3(1-z)), wherein z is greater than 0 or lessthan 1. z may be as further defined hereinbefore.

The perovskite semiconductor employed in the optoelectronic device ofthe invention may comprise said mixed-anion perovskite and asingle-anion perovskite, for instance in a blend, wherein said singleanion perovskite comprises a first cation, a second cation and an anionselected from halide anions and chalcogenide anions; wherein the firstand second cations are as herein defined for said mixed-anionperovskite. For instance, the optoelectronic device may comprise:CH₃NH₃PbICl₂ and CH₃NH₃PbI₃; CH₃NH₃PbICl₂ and CH₃NH₃PbBr₃; CH₃NH₃PbBrCl₂and CH₃NH₃PbI₃; or CH₃NH₃PbBrCl₂ and CH₃NH₃PbBr₃.

The optoelectronic device may comprise a perovskite of formula(H₂N=CH-NH₂)PbI_(3z)Br_(3(1-z)), wherein z is as defined herein, and asingle-anion perovskite such as (H₂N═CH—NH₂)PbI₃ or (H₂N═CH—NH₂)PbBr₃.

Alternatively, the perovskite semiconductor employed in theoptoelectronic device of the invention may comprise more than oneperovskite, wherein each perovskite is a mixed-anion perovskite, andwherein said mixed-anion perovskite is as herein defined. For instance,the optoelectronic device may comprise two or three said perovskites.The optoelectronic device of the invention may, for instance, comprisetwo perovskites wherein both perovskites are mixed-anion perovskites.For instance, the optoelectronic device may comprise: CH₃NH₃PbICl₂ andCH₃NH₃PbIBr₂; CH₃NH₃PbICl₂ and CH₃NH₃PbBrI₂; CH₃NH₃PbBrCl₂ andCH₃NH₃PbIBr₂; or CH₃NH₃PbBrCl₂ and CH₃NH₃PbIBr₂.

The optoelectronic device may comprise two different perovskites,wherein each perovskite is a perovskite of formula(H₂N-CH-NH₂)PbI₃Br_(3(1-z)), wherein z is as defined herein.

In some embodiments of the optoelectronic device of the invention, when[B] is a single metal cation which is Pb²⁺, one of said two or moredifferent halide anions is iodide or fluoride; and when [B] is a singlemetal cation which is Sn²⁺ one of said two or more different halideanions is fluoride. Usually, in some embodiments of the optoelectronicdevice of the invention, one of said two or more different halide anionsis iodide or fluoride. Typically, in some embodiments of theoptoelectronic device of the invention, one of said two or moredifferent halide anions is iodide and another of said two or moredifferent halide anions is fluoride or chloride. Often, in someembodiments of the optoelectronic device of the invention, one of saidtwo or more different halide anions is fluoride. Typically, in someembodiments of the optoelectronic device of the invention, either: (a)one of said two or more different anions is fluoride and another of saidsaid two or more different anions is chloride, bromide or iodide; or (b)one of said two or more different anions is iodide and another of saidtwo or more different anions is fluoride or chloride. Typically, [X] istwo different halide anions X and X′. Often, in the optoelectronicdevice of the invention, said divalent metal cation is Sn²⁺.Alternatively, in the optoelectronic device of the invention, saiddivalent metal cation may be Pb²⁺.

The n-type region in the optoelectronic device of the inventioncomprises one or more n-type layers. Often, the n-type region is ann-type layer, i.e. a single n-type layer. In other embodiments, however,the n-type region may comprise an n-type layer and an n-type excitonblocking layer. In cases where an n-type exciton blocking layer isemployed, the n-type exciton blocking layer is usually disposed betweenthe n-type layer and the layer(s) comprising the perovskitesemiconductor.

An exciton blocking layer is a material which is of wider bad gap thanthe perovskite, but has either its conduction band or valance bandclosely matched with those of the perovskite. If the conduction band (orlowest unoccupied molecular orbital energy levels) of the excitonblocking layer are closely aligned with the conduction band of theperovskite, then electrons can pass from the perovskite into and throughthe exciton blocking layer, or through the exciton blocking layer andinto the perovskite, and we term this an n-type exciton blocking layer.An example of such is bathocuproine, as described in {P. Peumans, A.Yakimov, and S. R. Forrest, “Small molecular weight organic thin-filmphotodetectors and solar cells” J. Appl. Phys. 93, 3693 (2001)} and{Masaya Hirade, and Chihaya Adachi, “Small molecular organicphotovoltaic cells with exciton blocking layer at anode interface forimproved device performance” Appl. Phys. Lett. 99, 153302 (2011)}}.

An n-type layer is a layer of an electron-transporting (i.e. an n-type)material. The n-type material may be a single n-type compound orelemental material, or a mixture of two or more n-type compounds orelemental materials, which may be undoped or doped with one or moredopant elements.

The n-type layer employed in the optoelectronic device of the inventionmay comprise an inorganic or an organic n-type material.

A suitable inorganic n-type material may be selected from a metal oxide,a metal sulphide, a metal selenide, a metal telluride, a perovskite,amorphous Si, an n-type group IV semiconductor, an n-type group III-Vsemiconductor, an n-type group II-VI semiconductor, an n-type groupI-VII semiconductor, an n-type group IV-VI semiconductor, an n-typegroup V-VI semiconductor, and an n-type group II-V semiconductor, any ofwhich may be doped or undoped.

The n-type material may be selected from a metal oxide, a metalsulphide, a metal selenide, a metal telluride, amorphous Si, an n-typegroup IV semiconductor, an n-type group III-V semiconductor, an n-typegroup II-VI semiconductor, an n-type group I-VII semiconductor, ann-type group IV-VI semiconductor, an n-type group V-VI semiconductor,and an n-type group II-V semiconductor, any of which may be doped orundoped.

More typically, the n-type material is selected from a metal oxide, ametal sulphide, a metal selenide, and a metal telluride.

Thus, the n-type layer may comprise an inorganic material selected fromoxide of titanium, tin, zinc, niobium, tantalum, tungsten, indium,gallium, neodinium, palladium, or cadmium, or an oxide of a mixture oftwo or more of said metals. For instance, the n-type layer may compriseTiO₂, SnO₂, ZnO, Nb₂O₅, Ta₂O₅, WO₃, W₂O₅, In₂O₃, Ga₂O₃, Nd₂O₃, PbO, orCdO.

Other suitable n-type materials that may be employed include sulphidesof cadmium, tin, copper, or zinc, including sulphides of a mixture oftwo or more of said metals. For instance, the sulphide may be FeS₂, CdS,ZnS or Cu₂ZnSnS₄.

The n-type layer may for instance comprise a selenide of cadmium, zinc,indium, or gallium or a selenide of a mixture of two or more of saidmetals; or a telluride of cadmium, zinc, cadmium or tin, or a tellurideof a mixture of two or more of said metals. For instance, the selenidemay be Cu(In,Ga)Se₂. Typically, the telluride is a telluride of cadmium,zinc, cadmium or tin. For instance, the telluride may be CdTe.

The n-type layer may for instance comprise an inorganic materialselected from oxide of titanium, tin, zinc, niobium, tantalum, tungsten,indium, gallium, neodinium, palladium, cadmium, or an oxide of a mixtureof two or more of said metals; a sulphide of cadmium, tin, copper, zincor a sulphide of a mixture of two or more of said metals; a selenide ofcadmium, zinc, indium, gallium or a selenide of a mixture of two or moreof said metals; or a telluride of cadmium, zinc, cadmium or tin, or atelluride of a mixture of two or more of said metals.

Examples of other semiconductors that may be suitable n-type materials,for instance if they are n-doped, include group IV compoundsemiconductors; amorphous Si; group III-V semiconductors (e.g. galliumarsenide); group II-VI semiconductors (e.g. cadmium selenide); groupI-VII semiconductors (e.g. cuprous chloride); group IV-VI semiconductors(e.g. lead selenide); group V-VI semiconductors (e.g. bismuthtelluride); and group II-V semiconductors (e.g. cadmium arsenide).

Typically, the n-type layer comprises TiO₂.

When the n-type layer is an inorganic material, for instance TiO₂ or anyof the other materials listed above, it may be a compact layer of saidinorganic material. Preferably the n-type layer is a compact layer ofTiO₂.

Other n-type materials may also be employed, including organic andpolymeric electron-transporting materials, and electrolytes. Suitableexamples include, but are not limited to a fullerene or a fullerenederivative, an organic electron transporting material comprisingperylene or a derivative thereof, or poly{[N,N0-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,50-(2,20-bithiophene)}(P(NDI2OD-T2)).

The p-type region in the optoelectronic device of the inventioncomprises one or more p-type layers. Often, the p-type region is ap-type layer, i.e. a single p-type layer. In other embodiments, however,the p-type region may comprise a p-type layer and a p-type excitonblocking layer. In cases where a p-type exciton blocking layer isemployed, the p-type exciton blocking layer is usually disposed betweenthe p-type layer and the layer(s) comprising the perovskitesemiconductor. If the valance band (or highest occupied molecularorbital energy levels) of the exciton blocking layer is closely alignedwith the valance band of the perovskite, then holes can pass from theperovskite into and through the exciton blocking layer, or through theexciton blocking layer and into the perovskite, and we term this ap-type exciton blocking layer. An example of such istris[4-(5-phenylthiophen-2-yl)phenyl]amine, as described in {MasayaHirade, and Chihaya Adachi, “Small molecular organic photovoltaic cellswith exciton blocking layer at anode interface for improved deviceperformance” Appl. Phys. Lett. 99, 153302 (2011)}.

A p-type layer is a layer of a hole-transporting (i.e. a p-type)material. The p-type material may be a single p-type compound orelemental material, or a mixture of two or more p-type compounds orelemental materials, which may be undoped or doped with one or moredopant elements.

The p-type layer employed in the optoelectronic device of the inventionmay comprise an inorganic or an organic p-type material.

Suitable p-type materials may be selected from polymeric or molecularhole transporters. The p-type layer employed in the optoelectronicdevice of the invention may for instance comprise spiro-OMeTAD(2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene)),P3HT (poly(3-hexylthiophene)), PCPDTBT(Poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]]),PVK (poly(N-vinylcarbazole)), HTM-TFSI (1-hexyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide), Li-TFSI (lithiumbis(trifluoromethanesulfonyl)imide) or tBP (tert-butylpyridine).Usually, the p-type material is selected from spiro-OMeTAD, P3HT,PCPDTBT and PVK. Preferably, the p-type layer employed in theoptoelectronic device of the invention comprises spiro-OMeTAD.

The p-type layer may for example comprise spiro-OMeTAD(2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene)),P3HT (poly(3-hexylthiophene)), PCPDTBT(Poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,l-b:3,4-b′]dithiophene-2,6-diyl]]),or PVK (poly(N-vinylcarbazole)).

Suitable p-type materials also include molecular hole transporters,polymeric hole transporters and copolymer hole transporters. The p-typematerial may for instance be a molecular hole transporting material, apolymer or copolymer comprising one or more of the following moieties:thiophenyl, phenelenyl, dithiazolyl, benzothiazolyl,diketopyrrolopyrrolyl, ethoxydithiophenyl, amino, triphenyl amino,carbozolyl, ethylene dioxythiophenyl, dioxythiophenyl, or fluorenyl.Thus, the p-type layer employed in the optoelectronic device of theinvention may for instance comprise any of the aforementioned molecularhole transporting materials, polymers or copolymers.

Suitable p-type materials also include m-MTDATA(4,4′,4″-tris(methylphenylphenylamino)triphenylamine), MeOTPD(N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine), BP2T(5,5′-di(biphenyl-4-yl)-2,2′-bithiophene), Di-NPB(N,N′—Di—[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine),α-NPB (N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine), TNATA(4,4′,4″-tris-(N-(naphthylen-2-yl)-N-phenylamine)triphenylamine), BPAPF(9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene), spiro-NPB(N2,N7-Di-1-naphthalenyl-N2,N7-diphenyl-9,9′-spirobi[9H-fluorene]-2,7-diamine),4P-TPD (4,4′-bis-(N,N-diphenylamino)-tetraphenyl), PEDOT:PSS andspiro-OMeTAD.

The p-type layer may be doped with an ionic salt or a base. The p-typelayer may for instance be doped with an ionic salt selected fromHMI-TFSI (1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide)and Li-TFSI (lithium bis(trifluoromethanesulfonyl)imide), or with a basewhich is tBP (tert-butylpyridine).

Additionally or alternatively, the p-type layer may be doped to increasethe hole-density. The p-type layer may for instance be doped with NOBF₄(Nitrosonium tetrafluoroborate), to increase the hole-density.

In other embodiments, the p-type layer may comprise an inorganic holetransporter. For instance, the p-type layer may comprise an inorganichole transporter comprising an oxide of nickel, vanadium, copper ormolybdenum; CuI, CuBr, CuSCN, Cu₂O, CuO or CIS; a perovskite; amorphousSi; a p-type group IV semiconductor, a p-type group III-V semiconductor,a p-type group II-VI semiconductor, a p-type group I-VII semiconductor,a p-type group IV-VI semiconductor, a p-type group V-VI semiconductor,and a p-type group II-V semiconductor, which inorganic material may bedoped or undoped. The p-type layer may be a compact layer of saidinorganic hole transporter.

The p-type layer may for instance comprise an inorganic hole transportercomprising an oxide of nickel, vanadium, copper or molybdenum; CuI,CuBr, CuSCN, Cu₂O, CuO or CIS; amorphous Si; a p-type group IVsemiconductor, a p-type group III-V semiconductor, a p-type group II-VIsemiconductor, a p-type group I-VII semiconductor, a p-type group IV-VIsemiconductor, a p-type group V-VI semiconductor, and a p-type groupII-V semiconductor, which inorganic material may be doped or undoped.The p-type layer may for instance comprise an inorganic hole transporterselected from CuI, CuBr, CuSCN, Cu₂O, CuO and CIS. The p-type layer maybe a compact layer of said inorganic hole transporter.

Typically, the p-type layer comprises a polymeric or molecular holetransporter, and the n-type layer comprises an inorganic n-typematerial. The p-type polymeric or molecular hole transporter may be anysuitable polymeric or molecular hole transporter, for instance any ofthose listed above. Likewise, the inorganic n-type material may be anysuitable n-type inorganic, for instance any of those listed above. Inone embodiment, for instance, the p-type layer comprises spiro-OMeTADand the n-type layer comprises TiO₂. Typically, in that embodiment, then-type layer which comprises TiO2 is a compact layer of TiO₂.

In other embodiments, both the n-type layer and the p-type layercomprise inorganic materials. Thus, the n-type layer may comprise aninorganic n-type material and the p-type layer may comprise an inorganicp-type material. The inorganic p-type material may be any suitablep-type inorganic, for instance any of those listed above. Likewise, theinorganic n-type material may be any suitable n-type inorganic, forinstance any of those listed above.

In yet other embodiments, the p-type layer comprises an inorganic p-typematerial (i.e. an inorganic hole transporter) and the n-type layercomprises a polymeric or molecular hole transporter. The inorganicp-type material may be any suitable p-type inorganic, for instance anyof those listed above. Likewise, the n-type polymeric or molecular holetransporter may be any suitable n-type polymeric or molecular holetransporter, for instance any of those listed above.

For instance, the p-type layer may comprise an inorganic holetransporter and the n-type layer may comprise an electron transportingmaterial, wherein the electron transporting material comprises afullerene or a fullerene derivative, an electrolyte, or an organicelectron transporting material, preferably wherein the organic electrontransporting material comprises perylene or a derivative thereof, orpoly{[N,N0-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,50-(2,20-bithiophene)}(P(NDI2OD-T2)). The inorganic hole transporter may for instance comprisean oxide of nickel, vanadium, copper or molybdenum; CuI, CuBr, CuSCN,Cu₂O, CuO or CIS; a perovskite; amorphous Si; a p-type group IVsemiconductor, a p-type group III-V semiconductor, a p-type group II-VIsemiconductor, a p-type group I-VII semiconductor, a p-type group IV-VIsemiconductor, a p-type group V-VI semiconductor, and a p-type groupII-V semiconductor, which inorganic material may be doped or undoped.More typically, the inorganic hole transporter comprises an oxide ofnickel, vanadium, copper or molybdenum; CuI, CuBr, CuSCN, Cu₂O, CuO orCIS; a p-type group IV semiconductor, a p-type group III-Vsemiconductor, a p-type group II-VI semiconductor, a p-type group I-VIIsemiconductor, a p-type group IV-VI semiconductor, a p-type group V-VIsemiconductor, and a p-type group II-V semiconductor, which inorganicmaterial may be doped or undoped. Thus, the inorganic hole transportermay comprise an oxide of nickel, vanadium, copper or molybdenum; CuI,CuBr, CuSCN, Cu₂O, CuO or CIS.

Paragraphs which follow concern the use of a second, p-type perovskite,in the p-type layer, or a second, n-type perovskite, in the n-typelayer. (In preferred embodiments, though, neither the p-type layer northe n-type layer comprises a perovskite. Thus, preferably, neither thep-type region nor the n-type region comprises a perovskite.)

When the p-type layer comprises an inorganic hole transporter which is aperovskite, the perovskite is different from the perovskite used in saidlayer of a perovskite semiconductor without open porosity, and, whenpresent, in said “first layer” which also comprises the scaffoldmaterial. Thus, when the p-type layer comprises an inorganic holetransporter which is a perovskite, the perovskite of the p-type layer istermed herein a “second perovskite” (and the perovskite in said layer ofa perovskite semiconductor without open porosity, and, when present, insaid first layer, is referred to herein as the “first perovskite”).

Similarly, when the n-type layer comprises an inorganic electrontransporter which is a perovskite, the perovskite will be different fromthe perovskite used in said layer of a perovskite semiconductor withoutopen porosity, and, when present, in said “first layer” which alsocomprises the scaffold material. Thus, when the n-type layer comprisesan inorganic electron transporter which is a perovskite, the perovskiteis herein termed a “second perovskite” (and the perovskite in said layerof a perovskite semiconductor without open porosity, and, when present,in said “first layer”, is referred to herein as the “first perovskite”).

The skilled person will appreciate that the addition of a doping agentto a perovskite may be used to control the charge transfer properties ofthat perovskite. Thus, for instance, a perovskite that is an intrinsicmaterial may be doped to form an n-type or a p-type material.Accordingly, the first perovskite and/or the second perovskite maycomprise one or more doping agents. Typically the doping agent is adopant element.

The addition of different doping agents to different samples of the samematerial may result in the different samples having different chargetransfer properties. For instance, the addition of one doping agent to afirst sample of perovskite material may result in the first samplebecoming an n-type material, whilst the addition of a different dopingagent to a second sample of the same perovskite material may result inthe second sample becoming a p-type material.

Thus, at least one of the first and second perovskites may comprise adoping agent. The first perovskite may for instance comprise a dopingagent that is not present in the or each second perovskite. Additionallyor alternatively, the, or one of the, second perovskites, may comprise adoping agent that is not present in the first perovskite. Thus thedifference between the first and second perovskites may be the presenceor absence of a doping agent, or it may be the use of a different dopingagent in each perovskite. Alternatively, the difference between thefirst and second perovskites may not lie in the doping agent but insteadthe difference may lie in the overall structure of the first and secondperovskites.

The second perovskite, when present, may be a perovskite comprising afirst cation, a second cation, and at least one anion.

In some embodiments, the second perovskite which is employed in thep-type or the n-type layer, which is different from the firstperovskite, is a perovskite compound of formula (IB):

wherein:

-   [A] is at least one organic cation or at least one Group I metal    cation;-   [B] is at least one metal cation; and-   [X] is at least one anion.

As the skilled person will appreciate, [A] may comprise Cs⁺.

Usually, [B] comprises Pb²⁺ or Sn²⁺. More typically, [B] comprises Pb²⁺.

Typically, [X] comprises a halide anion or a plurality of differenthalide anions.

Usually, [X] comprises I⁻.

In some embodiments, [X] is two or more different anions, for instance,two or more different halide anions. For instance, [X] may comprise I⁻and F⁻, I⁻ and Br⁻ or I⁻ and C1⁻.

Usually, the perovskite compound of formula IB is CsPbI₃ or CsSnI₃. Forinstance, the perovskite compound of formula (IB) may be CsPbI₃.

Alternatively, the perovskite compound of formula (IB) may be CsPbI₂Cl,CsPbICl₂, CsPbI₂F, CsPbIF₂, CsPbI₂Br, CsPbIBr₂, CsSnI₂Cl, CsSnICl₂,CsSnI₂F, CsSnIF₂, CsSnI₂Br or CsSnIBr₂. For instance, the perovskitecompound of formula (IB) may be CsPbI₂Cl or CsPbICl₂. Typically, theperovskite compound of formula (IB) is CsPbICl₂.

In the perovskite compound of formula (IB): [X] may be one, two or moredifferent anions as defined herein, for instance, two or more differentanions as defined herein for the first perovskite; [A] usually comprisesan organic cation as defined herein, as above for the first perovskite;and [B] typically comprises a metal cation as defined herein. The metalcation may be defined as hereinbefore for the first perovskite.

In some embodiments, the second perovskite is a perovskite as definedfor the first perovskite hereinabove, provided that the secondperovskite is different from the first perovskite.

The scaffold material which is employed in the embodiments of theoptoelectronic device of the invention which comprise said first layer,may be a dielectric scaffold material. Usually, the dielectric scaffoldmaterial has a band gap of equal to or greater than 4.0 eV.

Usually, in the optoelectronic device of the invention, the dielectricscaffold material comprises an oxide of aluminium, zirconium, silicon,yttrium or ytterbium. For instance, the dielectric scaffold material maycomprise zirconium oxide, silica, alumina, ytterbium oxide or yttriumoxide; or alumina silicate. Often, dielectric scaffold materialcomprises silica, or alumina. More typically, the dielectric scaffoldmaterial comprises porous alumina.

Typically, in the optoelectronic device of the invention, the dielectricscaffold material is mesoporous. Thus, typically, in the optoelectronicdevice of the invention, the dielectric scaffold material comprisesmesoporous alumina.

Alternatively, the scaffold material may be an inorganicelectron-transporting material, such as, for instance, titania. Thus,for instance, the scaffold material may comprise an oxide of titanium,tin, zinc, niobium, tantalum, tungsten, indium, gallium, neodinium,palladium or cadmium. For instance, the scaffold material may compriseTiO₂, SnO₂, ZnO, Nb₂O₅, Ta₂O₅, WO₃, W₂O₅, In₂O₃, Ga₂O₃, Nd₂O₃, PbO, orCdO. Often, the scaffold material comprises a mesoporous oxide oftitanium, tin, zinc, niobium, tantalum, tungsten, indium, gallium,neodinium, palladium or cadmium or a mixture thereof. Titania, poroustitania, and mesoporous titania are preferred. Typically, in suchembodiments, the scaffold material comprises porous titania, preferablymesoporous titania.

The scaffold material may for instance comprise an inorganic holetransporting material.

The scaffold material may on the other hand be an inorganic holetransporting material. Thus, the scaffold material may for instancecomprise an oxide of nickel, vanadium, copper or molybdenum, CuI, CuBr,CuSCN, Cu₂O, CuO or CIS.

The porosity of the scaffold material employed in the embodiments of theoptoelectronic device of the invention which comprise said first layer,is usually equal to or greater than 50%. For instance, the porosity maybe about 70%. In one embodiment, the porosity is equal to or greaterthan 60%, for instance equal to or greater than 70%.

Typically, in the optoelectronic device of the invention, the thicknessof the photoactive region is from 100 nm to 3000 nm, for instance from200 nm to 1000 nm, or for instance the thickness may be from 300 nm to800 nm. Often, thickness of the photoactive layer is from 400 nm to 600nm. Usually the thickness is about 500 nm.

The optoelectronic device of the invention usually comprises a firstelectrode and a second electrode. Thus, the optoelectronic device of theinvention typically comprises a first electrode, a second electrode,and, disposed between the first and second electrodes, said photoactiveregion.

The first and second electrodes are an anode and a cathode, and usuallyone or both of the anode and cathode is transparent to allow the ingressof light. At least one of the electrodes is usually semi-transparentacross the visible to near Infrared region of the solar spectrum.Semi-transparent is with a transparency of typically 80%, and rangingfrom 40 to 90%. The choice of the first and second electrodes of theoptoelectronic devices of the present invention may depend on thestructure type. Typically, the first layer of the device is depositedonto the first electrode which comprises tin oxide, more typically ontoa fluorine-doped tin oxide (FTO) anode, which is usually a transparentor semi-transparent material. Thus, the first electrode is usuallytransparent and typically comprises tin oxide, more typicallyfluorine-doped tin oxide (FTO). Usually, the thickness of the firstelectrode is from 200 nm to 600 nm, more typically from 300 to 500 nm.For instance the thickness may be 400 nm. Typically, FTO is coated ontoa glass sheet. Usually, the second electrode comprises a high workfunction metal, for instance gold, silver, nickel, palladium orplatinum, and typically silver. Usually, the thickness of the secondelectrode is from 50 nm to 250 nm, more usually from 100 nm to 200 nm.For instance, the thickness of the second electrode may be 150 nm.

Often, the first electrode will comprise a transparent orsemi-transparent electrically conductive material. For instance, thefirst electrode may comprise a transparent conducting oxide. Transparentconducting oxides include tin oxide, zinc oxide, doped tin oxide anddoped zinc oxide. For example, the first electrode may comprise ITO(indium tin oxide), FTO (fluorine-doped tin oxide) or AZO(aluminium-doped tin oxide), preferably FTO. The first electrode maycomprise from 90 to 100 wt% of ITO, FTO or AZO, and in some cases thefirst electrode may consist essentially of ITO, FTO or AZO. Usually, thethickness of the first electrode is from 200 nm to 600 nm, moretypically from 300 to 500 nm. For instance, the thickness may be 400 nm.The first electrode will often be disposed on a glass substrate. Forinstance, the first electrode may comprise FTO and may be disposed on aglass substrate. In the optoelectronic devices of the invention, theingress and/or egress of light typically occurs through the firstelectrode as it is often transparent or semi-transparent. It is possiblefor light to enter a device through a metal electrode (such as thesecond electrode may often be), particularly if the metal electrodeforms a thin layer.

Often, the second electrode comprises a metal. Usually, the secondelectrode comprises a high work function metal, for instance aluminium,gold, silver, nickel, palladium or platinum, and typically silver orgold. Usually, the thickness of the second electrode is from 50 nm to250 nm, more usually from 100 nm to 200 nm. For instance, the thicknessof the second electrode may be 150 nm.

In one embodiment of the invention, the optoelectronic device of theinvention may comprise a first electrode, a second electrode, and,disposed between the first and second electrodes, said photoactiveregion; wherein the first electrode is in contact with the n-type regionof said photoactive region and the second electrode is in contact withthe p-type region of said photoactive region.

Accordingly, the optoelectronic device according to the invention maycomprise the following regions in the following order:

-   I. a first electrode;-   II. an n-type region comprising at least one n-type layer;-   III. a layer of a perovskite semiconductor without open porosity;-   IV. a p-type region comprising at least one p-type layer; and-   V. a second electrode.

The term “the following regions in the following order”, as used herein,means that each of the regions listed will be present, and that theordering of each of the present layers will be in the given order. Forinstance, in the above case (I, II, III, IV, V), II succeeds I, andpreceeds III, and II alone is between I and III (i.e. neither IV nor Vare between I and III, but II is). This is the normal understanding of“in the following order”. The order does not, however, define theorientation in space of the collection of regions: I, II, III isequivalent to III, II, I (i.e. “up” and “down” or “left” and “right” areundefined). Additional layers or regions may be present between each ofthese regions. For instance, I, II, III includes I, Ia, II, IIa, III andI, Ia, Ib, II, III. Typically, however, each region (e.g. I to V) is incontact with both the preceding and the succeeding region.

Additional layers or regions may be present between each of theseregions. Typically, however, each region I to V is in contact with boththe preceding and the succeeding region. Each of the regions (a firstelectrode, an n-type region, a layer of a perovskite semiconductorwithout open porosity, a p-type region and a second electrode) may be asdefined anywhere herein. For example, the optoelectronic deviceaccording to the invention may comprise the following regions in thefollowing order:

-   I. a first electrode which comprises a transparent conducting oxide,    preferably FTO;-   II. an n-type region comprising at least one n-type layer;-   III. a layer of a perovskite semiconductor without open porosity;-   IV. a p-type region comprising at least one p-type layer; and-   V. a second electrode which comprises a metal, preferably silver or    gold.

In some embodiments, the second electrode may alternatively comprise atransparent conducting oxide. For example both the first electrode andthe second electrode may be selected from ITO, FTO and AZO. If thesecond electrode comprises a metal such as silver or gold, the thicknessof the second electrode may occasionally be from 1 to 10 nm. Forinstance, the first electrode may comprise FTO or ITO and the secondelectrode may comprise a layer of silver with a thickness of from 1 to10 nm, for instance 5 to 10 nm. A thin layer of silver can besemi-transparent.

The invention also provides an inverted heterojunction thin filmperovskite device. Therefore, in one embodiment, the optoelectronicdevice of the invention may comprise a first electrode, a secondelectrode, and, disposed between the first and second electrodes, saidphotoactive region; wherein the second electrode is in contact with then-type region of said photoactive region and the first electrode is incontact with the p-type region of said photoactive region. Such anarchitecture leads to what is known as an inverted device. These devicesmay have the configuration shown schematically in FIG. 8 . In somecircumstances it is desirable to have an inverted device structurewherein holes are collected through the substrate side of the device. Inparticular, inverted device architectures may be required for tandemapplications. Tandem applications include use with a number of inorganicphotovoltaic low band gap cells such as CIGS. The inventors havedeveloped a low temperature, ambient air and solution-processablephotovoltaic cell based on a semiconducting perovskite absorber. Often,the selective p-type and n-type contacts may be in the form of PEDOT:PSSand PC₆₀BM respectively. Remarkably, the final electrode configurationis very similar to that employed in “bulk heterojunction” polymer solarcells, albeit with the photoactive layer swapped where thebulk-heterojunction is replaced with a solid perovskite film and a veryrespectable 7.5% full sun power conversion efficiency is achieved withmuch scope for further improvement.

Thin-film photovoltaics based on solution-processable technologies offerthe promise of low-cost and easily manufacturable devices, necessary toassuage the world’s ever-increasing energy needs. Suitable candidatesare organic-based photovoltaics, inorganic and hybrid structures.Organic-based photovoltaics, while delivering low-cost and easilyprocessable technology, suffer from reduced performance in comparison toother thin film technologies due to fundamental losses in chargegeneration where a rather large offset between the donor and theacceptor is necessary to achieve efficient charge separation, limitingthe maximum achievable power conversion efficiency to just under 11 % ina single junction. Inorganic-based thin film photovoltaics can requirethe use of highly toxic solvents and high temperatures of over 500° C.,rendering them undesirable for mass-production.

For these reasons, perovskite-based hybrid photovoltaics are anappealing alternative as they can be processed under 150° C., are fullysolid-state and already exhibit high power conversion efficiencies ofover 12%. Perovskite absorbers have been previously used in sensitizedsolar cells as well as in thin-film architectures. Particularly, in thelatter configuration, the perovskite CH₃NH₃PbI_(3-x)Cl_(x) can act as acombined sensitizer and electron-transporter when processed on analumina mesostructured scaffold, minimizing energy losses simply becauseelectrons are directly transferred to the conductive substrate throughthe perovskite’s conduction band. In this way, extremely high opencircuit voltages of over 1.1 V can be achieved.

Often in perovskite-based photovoltaics, electrons are collected fromthe FTO substrate, while holes were collected at the metal cathode. Thisconfiguration is undesirable for some tandem applications where holesmust be collected at the TCO (transparent conducting oxide) interface.Here, a novel inverted device architecture is demonstrated. Often, it isbased on the n- and p-type materials commonly employed for chargecollection in organic photovoltaics, namely [6,6]-Phenyl C61 butyricacid methyl ester (PC_(6o)BM) and poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS), as well as V₂O₅ and NiO.

In one embodiment, the optoelectronic device of the invention comprisesa first electrode, a second electrode, and, disposed between the firstand second electrodes, said photoactive region; wherein the secondelectrode is in contact with the n-type region of said photoactiveregion and the first electrode is in contact with the p-type region ofsaid photoactive region, wherein the first electrode comprises atransparent or semitransparent electrically conductive material, andwherein the second electrode comprises aluminium, gold, silver, nickel,palladium or platinum.

Accordingly, the optoelectronic device according to the invention maycomprise the following regions in the following order:

-   I. a second electrode;-   II. an n-type region comprising at least one n-type layer;-   III. a layer of a perovskite semiconductor without open porosity;-   IV. a p-type region comprising at least one p-type layer; and-   V. a first electrode.

Each of the regions (a second electrode, an n-type region, a layer of aperovskite semiconductor without open porosity, a p-type region and afirst electrode) may be as defined anywhere herein.

For example, the optoelectronic device according to the invention maycomprise the following regions in the following order:

-   I. a second electrode which comprises a metal;-   II. an n-type region comprising at least one n-type layer;-   III. a layer of a perovskite semiconductor without open porosity;-   IV. a p-type region comprising at least one p-type layer; and-   V. a first electrode which comprises a transparent conducting oxide.

For example, the optoelectronic device according to the invention maycomprise the following regions in the following order:

-   I. a second electrode which comprises a metal, preferably silver or    gold;-   II. an n-type region comprising at least one n-type layer;-   III. a layer of a perovskite semiconductor without open porosity;-   IV. a p-type region comprising at least one p-type layer; and-   V. a first electrode which comprises a transparent conducting oxide,    preferably FTO.

Any of the components in an inverted device according to the inventionmay be as defined anywhere herein. For instance, the perovskite may be aperovskite according to any one of formulae I, Ia, II or IIa above. Forinstance, the perovskite may be a perovskite compound selected fromCH₃NH3PbI₃, CH₃NH₃PbBr₃, CH₃NH₃PbCl₃, CH₃NH₃PbF₃, CH₃NH₃PbBrI₂,CH₃NH₃PbBrCl₂, CH₃NH₃PbIBr₂, CH₃NH₃PbICl₂, CH₃NH₃PbClBr₂, CH₃NH₃PbI₂Cl,CH₃NH₃SnBrI₂, CH₃NH₃SnBrCl₂, CH₃NH₃SnF₂Br, CH₃NH₃SnIBr₂, CH₃NH₃SnICl₂,CH₃NH₃SnF₂I, CH₃NH₃SnClBr₂, CH₃NH₃SnI₂Cl and CH₃NH₃SnF₂Cl. In someembodiments, the second electrode may alternatively comprise atransparent conducting oxide. For example both the first electrode andthe second electrode may be selected from ITO, FTO and AZO. If thesecond electrode comprises a metal such as silver or gold, the thicknessof the second electrode may occasionally be from 1 to 10 nm. Forinstance, the first electrode may comprise FTO or ITO and the secondelectrode may comprise a layer of silver with a thickness of from 1 to10 nm, for instance 5 to 10 nm. A thin layer of silver can besemi-transparent.

The n-type region in an inverted device may comprise at least one n-typelayer as defined anywhere herein for a standard, non-inverted device.For instance, an n-type layer may comprise TiO₂, SnO₂, ZnO, Nb₂O₅,Ta₂O₅, WO₃, W₂O₅, In₂O₃, Ga₂O₃, Nd₂O₃, PbO, or CdO. In one embodiment,the n-type region may comprise a compact layer of titanium dioxide.Often, the n-type region may comprise a compact layer of titaniumdioxide and a layer of [60]PCBM ([6,6]-phenyl-C₆₁-butyric acid methylester). When the n-type region comprises a layer of titanium dioxide anda layer of [60]PCBM, the compact layer of titanium oxide is typicallyadjacent to the second electrode and the layer of [60]PCBM is typicallyadjacent to the layer of a perovskite semiconductor without openporosity.

The p-type region in an inverted device may comprises at least onep-type layer as defined anywhere herein for a standard, non-inverteddevice. For instance, a p-type layer may comprise spiro-OMeTAD(2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene)),P3HT (poly(3-hexylthiophene)), PCPDTBT(Poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]]),PVK (poly(N-vinylcarbazole)), PEDOT (poly(3,4-ethylenedioxythiophene)),or PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)).Alternatively, the p-type layer may for instance comprise an inorganichole transporter comprising an oxide of nickel, vanadium, copper ormolybdenum. In particular, the p-type region may comprise a layer ofSpiro-OMeTAD and/or a layer of PEDOT:PSS. In one embodiment, the p-typeregion comprises a layer of PEDOT:PSS. If the p-type region comprises alayer of a p-type polymeric material (such as PEDOT, or PEDOT:PSS), thep-type layer may be crosslinked. The layer is crosslinked in order tolimit the extent to which it is dissolved in the perovskite precursorsolution during manufacture of the device, i.e. the polymer (e.g.PEDOT:PSS) is crosslinked in order to insolubilise it. For instance, thep-type region may comprise a p-type layer comprising a polymericmaterial wherein the p-type layer is crosslinked. Occasionally, thep-type region may comprises a layer of PEDOT:PSS wherein the layer iscrosslinked. The p-type layer may be crosslinked by using a lewis acid,for instance Fe³⁺. The p-type region may comprise a layer of PEDOT:PSSwherin the layer has been crosslinked using FeCl₃.

An optoelectronic device according to the invention may comprise thefollowing regions in the following order:

-   I. a second electrode which comprises a metal;-   II. an n-type region comprising a compact layer of titanium dioxide    and a layer of [60]PCBM;-   III. a layer of a perovskite semiconductor without open porosity;-   IV. a p-type region comprising a layer of PEDOT:PSS, optionally    wherein the layer is crosslinked; and-   V. a first electrode which comprises a transparent conducting oxide.

An optoelectronic device according to the invention may comprise thefollowing regions in the following order:

-   I. a second electrode which comprises a metal, preferably aluminium,    silver or gold;-   II. an n-type region comprising a compact layer of titanium dioxide    and a layer of [60]PCBM;-   III. a layer of a perovskite semiconductor without open porosity;-   IV. a p-type region comprising a layer of PEDOT:PSS, optionally    wherein the layer is crosslinked; and-   V. a first electrode which comprises a transparent conducting oxide,    preferably FTO.

For example, an optoelectronic device according to the invention maycomprise the following regions in the following order:

-   I. a second electrode which comprises aluminium;-   II. a compact layer of titanium dioxide;-   III. a layer of [60]PCBM;-   IV. a layer of a perovskite semiconductor without open porosity;-   V. a layer of crosslinked PEDOT:PSS; and-   VI. a first electrode which comprises FTO.

Said photoactive region may be the only photoactive region in the deviceand the optoelectronic device of the invention may therefore be a singlejunction device.

Alternatively, the optoelectronic device of the invention may be atandem junction optoelectronic device or a multi junction optoelectronicdevice.

Accordingly, the optoelectronic device may comprise a first electrode, asecond electrode, and, disposed between the first and second electrodes:

-   said photoactive region; and-   at least one other photoactive region.

The other photoactive region or regions may be the same as or differentfrom the photoactive region defined hereinbefore.

In some embodiments, the other photoactive region or regions are thesame as the photoactive region defined hereinbefore.

Thus, the optoelectronic device of the invention may comprise: a firstelectrode, a second electrode, and, disposed between the first andsecond electrodes: a plurality of said photoactive regions.

When the optoelectronic device of the invention is a tandem or multijunction device, as the skilled person will appreciate, it may compriseone or more tunnel junctions. Each tunnel junction is usually disposedbetween two photoactive regions.

A tandem junction optoelectronic device (or multi junctionoptoelectronic device) according to the invention may combine the hereindisclosed perovskite thin film technology with known technologies todeliver optimised performance.

An “all perovskite” multi-junction cell is very attractive, however evenwithout the need to develop new absorbers, the current system employingCH₃NH₃PbI_(3-x)Cl_(x) is already very well set to match with crystallinesilicon and other thin film technologies such as CIS, CIGS and CZTSSe,if used as a top cell in a tandem junction. There is potential toproduce optoelectronic devices with efficiencies in excess of 20%. Theremarkable aspect of this is that it doesn’t require a “quantum leap” inthe currently presented technology, simply a little optimization andeffective integration. There are many distinct advantages of“piggybacking” on existing technologies; the continuing drop in the costof existing PV becomes advantageous, the market should be much morewilling to adopt an “enhanced silicon technology” rather than an all newperovskite technology, and last but not least, a key challenge for thebroader PV community has been to develop a wide-gap top cell for siliconand thin film technologies. In FIGS. 16 and 17 , schematic illustrationsof possible tandem junction device configurations are given, forperovskite on c-Si and perovskite on conventional thin film.

In one embodiment, the invention provides an optoelectronic device whichcomprises a first electrode, a second electrode, and, disposed betweenthe first and second electrodes:

-   said photoactive region as defined hereinbefore; and-   at least one other photoactive region,-   wherein the at least one other photoactive region comprises at least    one layer of a semiconductor material.

The at least one other photoactive region may be at least one otherphotoactive region from photoactive regions used in conventional andknown optoelectronic and photovoltaic devices. For instance, it may be aphotoactive region from a crystalline silicon photovoltaic cell or aphotoactive region from a conventional thin film gallium arsenide, CIGS,CIS or CZTSSe photovoltaic device.

Often, a tandem optoelectronic device comprises the following regions inthe following order:

-   I. a first electrode;-   II. a first photoactive region as defined anywhere hereinbefore;-   III. a second photoactive region which comprises a layer of a    semiconductor material; and-   IV. a second electrode.

The semiconductor material in region III may be any semiconductormaterial. The term “semiconductor material”, as used herein, refers to amaterial with electrical conductivity intermediate in magnitude betweenthat of a conductor and an insulator. Typically, a semiconductormaterial is a material that has a conductivity of from 10³ to 10⁻⁸Scm⁻¹. Standard techniques such as a 4-point probe conductivitymeasurement may be used to measure conductivity. Examples ofsemiconductor materials include an oxide or chalcogenide of a metal ormetalloid element; a group IV compound; a compound comprising a groupIII element and a group V element; a compound comprising a group IIelement and a group VI element; a compound comprising a group I elementand a group VII element; a compound comprising a group IV element and agroup VI element; a compound comprising a group V element and a group VIelement; a compound comprising a group II element and a group V element;a ternary or quaternary compound semiconductor; a perovskitesemiconductor or an organic semiconductor. Typical examples ofsemiconductor materials include oxides of titanium, niobium, tin, zinc,cadmium, copper or lead; chalcogenides of antimony or bismuth; copperzinc tin sulphide; copper zinc tin selenide, copper zinc tin selenidesulphide, copper indium gallium selenide; and copper indium galliumdiselenide. Further examples are group IV compound semiconductors (e.g.silicon carbide); group III-V semiconductors (e.g. gallium arsenide);group II-VI semiconductors (e.g. cadmium selenide); group I-VIIsemiconductors (e.g. cuprous chloride); group IV-VI semiconductors (e.g.lead selenide); group V-VI semiconductors (e.g. bismuth telluride); andgroup II-V semiconductors (e.g. cadmium arsenide); ternary or quaternarysemiconductors (eg. copper indium selenide, copper indium galliumdiselenide, or copper zinc tin sulphide); perovskite semiconductormaterials (e.g. CH₃NH₃PbI₃ and CH₃NH₃PbI₂Cl); and organic semiconductormaterials (e.g. conjugated polymeric compounds including polymers suchas polyacetylenes, polyphenylenes and polythiophenes). Examples oforganic semiconductors include poly(3,4-ethylenedioxythiophene),2,2-7,7-tetrakis-N,N-di-p-methoxyphenylamine-9,9-spirobifluorene(spiro-OMeTAD) and conjugated organic polymers such as polyacetylenes,polyphenylenes, polythiophenes or polyanilines. Examples of materialswhich are not semiconductor materials include, for instance elementalmetals, which are of course conductors, and electrical insulators ordielectrics such as silica or calcite.

The term “oxide”, as used herein, refers to a compound comprising atleast one oxide ion (i.e. O²⁻) or divalent oxygen atom. It is to beunderstood that the terms “metal oxide” and “an oxide of a metalelement” used herein encompass both oxides comprising one metal, andalso mixed-metal oxides. For the avoidance of doubt, a mixed-metal oxiderefers to a single oxide compound comprising more than one metalelement. Examples of mixed-metal oxides include zinc tin oxide andindium tin oxide. Similarly, it is to be understood that the terms“metalloid oxide” and “an oxide of a metalloid element” used hereinencompass oxides comprising one metalloid element and alsomixed-metalloid oxides. For the avoidance of doubt, a mixed-metalloidoxide refers to a single oxide compound comprising more than onemetalloid element.

The tem “chalcogenide”, used herein, refers to a compound comprising atleast one of a sulphide, selenide or telluride ion (i.e. S²⁻, Se²⁻, orTe²⁻) or a divalent sulphur, selenium or tellurium atom. It is to beunderstood that the terms “metal chalcogenide” and “a chalcogenide of ametal element” encompass chalcogenides comprising one metal and alsomixed-metal chalcogenides. For the avoidance of doubt, a mixed-metalchalcogenide refers to a single chalcogenide compound comprising morethan one metal element. Similarly, it is to be understood that the terms“metalloid chalcogenide” and “a chalcogenide of a metalloid element”used herein encompass chalcogenides comprising one metalloid and alsomixed-metalloid chalcogenides. For the avoidance of doubt, amixed-metalloid chalcogenide refers to a single chalcogenide compoundcomprising more than one metalloid element.

Occasionally, the semiconductor material comprises an oxide orchalcogenide of a metal or metalloid element. For example, thesemiconductor material consists of an oxide or chalcogenide of a metalor metalloid element. For instance, the semiconductor material comprisesan oxide of titanium, niobium, tin, zinc, cadmium, copper or lead or anycombination thereof; or a chalcogenide of antimony, bismuth or cadmiumor any combination thereof. For instance the semiconductor material maycomprise zinc tin oxide; copper zinc tin sulphide; copper indium galliumselenide, or copper indium gallium diselenide.

In one embodiment the semiconductor material may be a dopedsemiconductor, where an impurity element is present at a concentrationranging between 0.01 to 40%. If the impurity element acts as an electrondonor, then the semiconductor material will be doped to become n-type,if the impurity element acts as an electron acceptor, then thesemiconductor material will be doped to become p-type. It is noted thatfor metal oxides doped with impurity metalloid elements which substitutethe primary metalloid element, if the valency of the dopant is largerthan the valency of the primary metalloid element then the metal oxidewill be n-type doped, if the valency of the dopant metalloid element islower than that of the primary metalloid element then the metal oxidewill be p-type doped. Any of the above mentioned elements can be used todope any of the above mentioned semiconductor materials to differentlevels of efficacy and effect.

Accordingly, in some cases the semiconductor material comprises an oxideor chalcogenide of a metal or metalloid element; a group IV compound; acompound comprising a group III element and a group V element; acompound comprising a group II element and a group VI element; acompound comprising a group I element and a group VII element; acompound comprising a group IV element and a group VI element; acompound comprising a group V element and a group VI element; a compoundcomprising a group II element and a group V element; a ternary orquaternary compound semiconductor; or an organic semiconductor.

Often, the semiconductor material comprises silicon; an oxide oftitanium, niobium, tin, zinc, cadmium, copper or lead; a chalcogenide ofantimony or bismuth; copper zinc tin sulphide; copper zinc tin selenide,copper zinc tin selenide sulphide, copper indium gallium selenide;copper indium gallium diselenide, silicon carbide, gallium arsenide,cadmium selenide, cuprous chloride, lead selenide, bismuth telluride, orcadmium arsenide. If the semiconductor material comprises silicon, thesilicon may be monocrystalline, polycrystalline or amorphous.

The photoactive region according to the invention may be in tandem witha traditional silicon solar cell. For instance, the semiconductormaterial may comprise a layer of crystalline silicon.

In some embodiments the optoelectronic device comprises the followingregions in the following order:

-   I. a first electrode;-   II. a first photoactive region as defined in anywhere herein;-   III. a layer (A) of a p-type semiconductor;-   IV. a first layer of an intrinsic semiconductor;-   V. a layer (B) of a p-type semiconductor or a layer (B) of an n-type    semiconductor;-   VI. a second layer of an intrinsic semiconductor;-   VII. a layer (C) of an n-type semiconductor; and-   VIII. a second electrode.

Occasionally, the optoelectronic device comprises the following regionsin the following order:

-   I. a first electrode;-   II. a first region;-   III. a layer of perovskite semiconductor without open porosity;-   IV. a third region;-   V. a layer (A) of a p-type semiconductor;-   VI. a first layer of an intrinsic semiconductor;-   VII. a layer (B) of a p-type semiconductor or a layer (B) of an    n-type semiconductor;-   VIII. a second layer of an intrinsic semiconductor;-   IX. a layer (C) of an n-type semiconductor; and-   X. a second electrode;

-   wherein the first region is an n-type region comprising at least one    n-type layer and the third region is a p-type region comprising at    least one p-type layer; or-   the first region is a p-type region comprising at least one p-type    layer and the third region is an n-type region comprising at least    one n-type layer.

Any of the components (e.g. the perovskite, the first region or thethird region) in this tandem device may be as defined anywhere herein.Any p-type, n-type or intrinsic semiconductor referred to may compriseany semiconductor defined herein which may be appropriately p-doped,n-doped or undoped.

Often, the first region is a p-type region comprising at least onep-type layer and the third region is an n-type region comprising atleast one n-type layer. Accordingly, the p-type layer will be adjacentto the first electrode, and the perovskite photoactive region accordingto the invention will be inverted. Typically, light which falls on thedevice is incident through the first electrode. The n-type regioncomprising at least one n-type layer may be as defined herein and/or thep-type region comprising at least one p-type layer may be as definedherein.

Often, in a tandem optoelectronic device according to the invention, thelayer (A) of a p-type semiconductor is a layer of p-type amorphoussilicon and/or the layer (C) of an n-type semiconductor is a layer ofn-type amorphous silicon. Typically, the layer (A) of a p-typesemiconductor is a layer of p-type amorphous silicon and the layer (C)of an n-type semiconductor is a layer of n-type amorphous silicon Often,the first layer of an intrinsic semiconductor is a layer of intrinsicamorphous silicon and/or the second layer of an intrinsic semiconductoris a layer of intrinsic amorphous silicon. Sometimes, the first layer ofan intrinsic semiconductor is a layer of intrinsic amorphous silicon andthe second layer of an intrinsic semiconductor is a layer of intrinsicamorphous silicon. In the tandem device, the layer (B) of a p-typesemiconductor or the layer (B) of an n-type semiconductor may be a layerof p-type crystalline silicon or a layer of n-type crystalline silicon.

As defined elsewhere herein, the first electrode typically comprises atransparent conducting oxide and/or the second electrode comprises ametal. Often the first electrode typically comprises a transparentconducting oxide and the second electrode comprises a metal. Thetransparent conducting oxide may be as defined above and is often FTO,ITO, or AZO, and typically ITO. The metal may be any metal. Generallythe second electrode comprises a metal selected from silver, gold,copper, aluminium, platinum, palladium, or tungsten. This list of metalsmay also apply to other instances of the second electrode herein. Often,the first electrode material comprises ITO and/or the second electrodecomprises silver. Typically, the first electrode material comprises ITOand the second electrode comprises silver.

Rather than the photoactive region according to the invention comprisinga layer of perovskite without open porosity being in tandem with asilicon photoactive region, it may be in tandem with a thin filmphotoactive region. For instance, the optoelectronic device maycomprises the following regions in the following order:

-   I. a first electrode;-   II. a first photoactive region as defined anywhere hereinbefore;-   III. a second photoactive region which comprises a layer of a    semiconductor material; and-   IV. a second electrode;

wherein the semiconductor material comprises a layer of copper zinc tinsulphide, copper zinc tin selenide, copper zinc tin selenide sulphide,copper indium gallium selenide, copper indium gallium diselenide orcopper indium selenide. The layer of a semiconductor material may be athin film of a semiconductor material.

In one embodiment, the optoelectronic device comprises the followingregions in the following order:

-   I. a first electrode;-   II. a first photoactive region as defined hereinbefore;-   III. a layer of a transparent conducting oxide;-   IV. a layer (D) of an n-type semiconductor;-   V. a layer of copper zinc tin sulphide, copper zinc tin selenide,    copper zinc tin selenide sulphide, copper indium gallium selenide,    copper indium gallium diselenide or copper indium selenide; and-   VI. a second electrode.

For example, the optoelectronic device according may comprises thefollowing regions in the following order:

-   I. a first electrode;-   II. a first region;-   III. a layer of perovskite semiconductor without open porosity;-   IV. a third region;-   V. a layer of a transparent conducting oxide;-   VI. a layer (D) of an n-type semiconductor;-   VII. a layer of copper zinc tin sulphide, copper zinc tin selenide,    copper zinc tin selenide sulphide, copper indium gallium selenide,    copper indium gallium diselenide or copper indium selenide; and-   VIII. a second electrode;

-   wherein the first region is an n-type region comprising at least one    n-type layer and the third region is a p-type region comprising at    least one p-type layer; or-   the first region is a p-type region comprising at least one p-type    layer and the third region is an n-type region comprising at least    one n-type layer.

The layer (D) of an n-type semiconductor may comprise any metal oxide orchalcogenide semiconductor. Often, the layer (D) of an n-typesemiconductor comprises cadmium sulfide.

Typically, in the tandem device comprising a thin film of asemiconductor, the first region is an n-type region comprising at leastone n-type layer and the third region is a p-type region comprising atleast one p-type layer. The n-type region comprising at least one n-typelayer may be as defined any where herein and/or the p-type regioncomprising at least one p-type layer may be as defined anywhere herein.

The first electrode and/or second electrode may be as defined above.Typically, the first electrode comprises a transparent conducting oxideand/or the second electrode comprises a metal. Often, the firstelectrode comprises a transparent conducting oxide and the secondelectrode comprises a metal. Typically, the first electrode comprisesITO and/or the second electrode comprises tungsten, or the firstelectrode comprises ITO and the second electrode comprises tungsten.

The optoelectronic device of the invention may be a photovoltaic device;a photodiode; a phototransistor; a photomultiplier; a photo resistor; aphoto detector; a light-sensitive detector; solid-state triode; abattery electrode; a light emitting device; a light emitting diode; atransistor; a solar cell; a laser; or a diode injection laser.

In a preferred embodiment, the optoelectronic device of the invention isa photovoltaic device, for instance a solar cell.

The optoelectronic device according to the invention may be a solarcell.

In another preferred embodiment, the optoelectronic device of theinvention is a light-emitting device, for instance a light emittingdiode.

The perovskite compounds employed in the optoelectronic device of theinvention, in said layer of a perovskite semiconductor without openporosity, and/or in said first layer, may be produced by a processcomprising mixing:

-   (a) a first compound comprising (i) a first cation and (ii) a first    anion; with-   (b) a second compound comprising (i) a second cation and (ii) a    second anion; wherein:    -   the first and second cations are as defined herein for the        perovskite; and    -   the first and second anions may be the same or different anions.

The perovskites which comprise at least one anion selected from halideanions and chalcogenide anions, may, for instance, be produced by aprocess comprising mixing:

-   (a) a first compound comprising (i) a first cation and (ii) a first    anion; with-   (b) a second compound comprising (i) a second cation and (ii) a    second anion; wherein:    -   the first and second cations are as herein defined for the        perovskite; and    -   the first and second anions may be the same or different anions        selected from halide anions and chalcogenide anions.

Typically, the first and second anions are different anions. Moretypically, the first and second anions are different anions selectedfrom halide anions.

The perovskite produced by the process may comprise further cations orfurther anions. For example, the perovskite may comprise two, three orfour different cations, or two, three of four different anions. Theprocess for producing the perovskite may therefore comprise mixingfurther compounds comprising a further cation or a further anion.Additionally or alternatively, the process for producing the perovskitemay comprise mixing (a) and (b) with: (c) a third compound comprising(i) the first cation and (ii) the second anion; or (d) a fourth compoundcomprising (i) the second cation and (ii) the first anion.

Typically, in the process for producing the perovskite, the secondcation in the mixed-anion perovskite is a metal cation. More typically,the second cation is a divalent metal cation. For instance, the secondcation may be selected from Ca²⁺, Sr²⁺, Cd²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Fe²⁺,Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ and Eu²⁺. Usually, the second cationis selected from Sn²⁺ and Pb²⁺.

Often, in the process for producing the perovskite, the first cation inthe mixed-anion perovskite is an organic cation.

Usually, the organic cation has the formula (R₁R₂R₃R₄N)⁺, wherein:

-   R₁ is hydrogen, or unsubstituted or substituted C₁-C₂₀ alkyl, or    unsubstituted or substituted aryl;-   R₂ is hydrogen, or unsubstituted or substituted C₁-C₂₀ alkyl, or    unsubstituted or substituted aryl;-   R₃ is hydrogen, or unsubstituted or substituted C₁-C₂₀ alkyl, or    unsubstituted or substituted aryl; and-   R₄ is hydrogen, or unsubstituted or substituted C₁-C₂₀ alkyl, or    unsubstituted or substituted aryl.

Mainly, in the organic cation, R₁ is hydrogen, methyl or ethyl, R₂ ishydrogen, methyl or ethyl, R₃ is hydrogen, methyl or ethyl, and R₄ ishydrogen, methyl or ethyl. For instance R₁ may be hydrogen or methyl, R₂may be hydrogen or methyl, R₃ may be hydrogen or methyl, and R₄ may behydrogen or methyl.

Alternatively, the organic cation may have the formula (R₅NH₃)⁺,wherein: R₅ is hydrogen, or unsubstituted or substituted C₁-C₂₀ alkyl.For instance, R₅ may be methyl or ethyl. Typically, R₅ is methyl.

Alternatively, the organic cation may have the formula(R₅R₆N═CH—NR₇R₈)⁺, wherein: R₅ is hydrogen, unsubstituted or substitutedC₁-C₂₀ alkyl, or unsubstituted or substituted aryl; R₆ is hydrogen,unsubstituted or substituted C₁-C₂₀ alkyl, or unsubstituted orsubstituted aryl; R₇ is hydrogen, unsubstituted or substituted C₁-C₂₀alkyl, or unsubstituted or substituted aryl; and R₈ is hydrogen,unsubstituted or substituted C₁-C₂₀ alkyl, or unsubstituted orsubstituted aryl.

Typically, R₅ in the cation (R₅R₆N═CH—NR₇R₈)⁺ is hydrogen, methyl orethyl, R₆ is hydrogen, methyl or ethyl, R₇ is hydrogen, methyl or ethyl,and R₈ is hydrogen, methyl or ethyl. For instance R₅ may be hydrogen ormethyl, R₆ may be hydrogen or methyl, R₇ may be hydrogen or methyl, andR₈ may be hydrogen or methyl.

The organic cation may, for example, have the formula (H₂N═CH—NH₂)⁺.

In the process for producing the perovskite, the perovskite is usually amixed-halide perovskite, wherein said two or more different anions aretwo or more different halide anions.

Typically, in the process for producing the perovskite, the perovskiteis a perovskite compound of the formula (I):

wherein:

-   [A] is at least one organic cation;-   [B] is at least one metal cation; and-   [X] is said two or more different anions; and

the process comprises mixing:

-   (a) a first compound comprising (i) a metal cation and (ii) a first    anion; with-   (b) a second compound comprising (i) an organic cation and (ii) a    second anion; wherein:    -   the first and second anions are different anions selected from        halide anions or chalcogenide anions.

The perovskite of the formula (I) may, for instance, comprise one, two,three or four different metal cations, typically one or two differentmetal cations. The perovskite of the formula (I), may, for instance,comprise one, two, three or four different organic cations, typicallyone or two different organic cations. The perovskite of the formula (I),may, for instance, comprise two, three or four different anions,typically two or three different anions. The process may, therefore,comprise mixing further compounds comprising a cation and an anion.

Typically, [X] is two or more different halide anions. The first andsecond anions are thus typically halide anions. Alternatively [X] may bethree different halide ions. Thus the process may comprise mixing athird compound with the first and second compound, wherein the thirdcompound comprises (i) a cation and (ii) a third halide anion, where thethird anion is a different halide anion from the first and second halideanions.

Often, in the process for producing the perovskite, the perovskite is aperovskite compound of the formula (IA):

wherein:

-   A is a organic cation;-   B is a metal cation; and-   [X] is said two or more different anions.

the process comprises mixing:

-   (a) a first compound comprising (i) a metal cation and (ii) a first    halide anion; with-   (b) a second compound comprising (i) an organic cation and (ii) a    second halide anion: wherein:    -   the first and second halide anions are different halide anions.

Usually, [X] is two or more different halide anions. Preferably, [X] istwo or three different halide anions. More preferably, [X] is twodifferent halide anions. In another embodiment [X] is three differenthalide anions.

Typically, in the process for producing the perovskite, the perovskiteis a perovskite compound of formula (II):

wherein:

-   A is an organic cation;-   B is a metal cation;-   X is a first halide anion;-   X′ is a second halide anion which is different from the first halide    anion; and-   y is from 0.05 to 2.95; and

the process comprises mixing:

-   (a) a first compound comprising (i) a metal cation and (ii) X; with-   (b) a second compound comprising (i) an organic cation and (ii) X′:    wherein the ratio of X to X′ in the mixture is equal to (3-y):y.

In order to achieve said ratio of X to X′ equal to (3-y):y, the processmay comprise mixing a further compound with the first and secondcompounds. For example, the process may comprise mixing a third compoundwith the first and second compounds, wherein the third compoundcomprises (i) the metal cation and (ii) X′. Alternative, the process maycomprising mixing a third compound with the first and second compounds,wherein the third compound comprises (i) the organic cation and (ii) X.

Usually, y is from 0.5 to 2.5, for instance from 0.75 to 2.25.Typically, y is from 1 to 2.

Typically, in the process for producing the perovskite, the firstcompound is BX₂ and the second compound is AX’.

Often the second compound is produced by reacting a compound of theformula (R₅NH₂), wherein: R₅ is hydrogen, or unsubstituted orsubstituted C₁-C₂₀ alkyl, with a compound of formula HX’. Typically, R₅may be methyl or ethyl, often R₅ is methyl.

Usually, the compound of formula (R₅NH₂) and the compound of formula HX’are reacted in a 1:1 molar ratio. Often, the reaction takes place undernitrogen atmosphere and usually in anhydrous ethanol. Typically, theanhydrous ethanol is about 200 proof. More typically from 15 to 30 ml ofthe compound of formula (R₅NH₂) is reacted with about 15 to 15 ml ofHX’, usually under nitrogen atmosphere in from 50 to 150 ml anhydrousethanol. The process may also comprise a step of recovering saidmixed-anion perovskite. A rotary evaporator is often used to extractcrystalline AX’.

Usually, the step of mixing the first and second compounds is a step ofdissolving the first and second compounds in a solvent. The first andsecond compounds may be dissolved in a ratio of from 1:20 to 20:1,typically a ratio of 1:1. Typically the solvent is dimethylformamide(DMF) or water. When the metal cation is Pb²⁺ the solvent is usuallydimethylformamide. When the metal cation is Sn²⁺ the solvent is usuallywater. The use of DMF or water as the solvent is advantageous as thesesolvents are not very volatile.

The perovskite semiconductor layer in the inventive devices can beprepared by solution-processing or by evaporation in vacuum. Reducedprocessing temperature is important for reducing the cost ofmanufacturing, enabling processing on plastic substrates and processingon top of other layers to enable the production of tandem andmulti-junction solar cells. Here, the inventors demonstrate that thedevices of the invention can operate with all layers processed atlow-temperature including the solution-processable scaffold.

The invention provides a process for producing an optoelectronic devicecomprising a photoactive region, which photoactive region comprises:

-   an n-type region comprising at least one n-type layer;-   a p-type region comprising at least one p-type layer; and, disposed    between the n-type region and the p-type region:-   a layer of a perovskite semiconductor without open porosity,

which process comprises:

-   (a) providing a first region;-   (b) disposing a second region on the first region, which second    region comprises a layer of a perovskite semiconductor without open    porosity; and-   (c) disposing a third region on the second region,

wherein:

-   the first region is an n-type region comprising at least one n-type    layer and the third region is a p-type region comprising at least    one p-type layer; or-   the first region is a p-type region comprising at least one p-type    layer and the third region is an n-type region comprising at least    one n-type layer.

Often, the first region is an n-type region comprising at least onen-type layer and the third region is a p-type region comprising at leastone p-type layer.

In the process of the invention, the n-type region, the n-type layer,the p-type region and the p-type layer may be as further defined hereinbefore for the optoelectronic device of the invention. Also, the layerof the perovskite semiconductor without open porosity, and theperovskite semiconductor itself, may be as further defined hereinbefore.

In one embodiment of the process of the invention, step (b), ofdisposing the second region on the first region, comprises:

producing a solid layer of the perovskite on the first region by vapourdeposition.

In this embodiment, the step of producing a solid layer by vapourdeposition typically comprises:

-   (i) exposing the first region to vapour, which vapour comprises said    perovskite or one or more reactants for producing said perovskite;    and-   (ii) allowing deposition of the vapour onto the first region, to    produce a solid layer of said perovskite thereon.

The perovskite in the vapour may be any of the perovskites discussedhereinbefore for the optoelectronic device of the invention, and istypically a perovskite of formula (I), (IA) or (II) as definedhereinbefore.

The one or more reactants for producing said perovskite may comprise thereactant types discussed above for the process for synthesising theperovskite compounds.

Thus, the one or more reactants may comprise:

-   (a) a first compound comprising (i) a first cation and (ii) a first    anion; and-   (b) a second compound comprising (i) a second cation and (ii) a    second anion,

as defined hereinbefore for the process for producing the perovskitecompounds employed in the optoelectronic device of the invention.

More particularly, the one or more reactants may comprise:

-   (a) a first compound comprising (i) a metal cation and (ii) a first    anion; with-   (b) a second compound comprising (i) an organic cation and (ii) a    second anion; wherein the first and second anions are different    anions selected from halide anions or chalcogenide anions, as    defined hereinbefore for the process for producing the perovskite    compounds employed in the optoelectronic device of the invention.

For instance, the one or more reactants may comprise:

-   (a) a first compound comprising (i) a metal cation and (ii) a first    halide anion; with-   (b) a second compound comprising (i) an organic cation and (ii) a    second halide anion; wherein the first and second halide anions are    different halide anions,

as defined hereinbefore for the process for producing the perovskitecompounds employed in the optoelectronic device of the invention.

For instance, when the perovskite being deposited is CH₃NH₃PbI₂Cl, theone or more reactants typically comprise (a) PbI₂, and (b) CH₃NH₃Cl.

The process generally further comprises producing the vapour in thefirst place by evaporating said perovskite or evaporating said one ormore reactants for producing said perovskite. In this step theperovskite or the one or more reactants for producing the perovskite aretypically transferred to an evaporation chamber which is subsequentlyevacuated. The perovskite or the one or more reactants for producing theperovskite are typically then heated.

The resulting vapour is then exposed to and thereby deposited the firstregion, to produce a solid layer of said perovskite thereon. Ifreactants are used, these react together in situ to produce theperovskite on the first region.

Typically, the vapour deposition is allowed to continue until the solidlayer of perovskite has a desired thickness, for instance a thickness offrom 10 nm to 100 µm, or more typically from 10 nm to 10 µm. Preferably,the vapour deposition is allowed to continue until the solid layer ofperovskite has a thickness of from 50 nm to 1000 nm, or for instancefrom 100 nm to 700 nm. For instance, deposition may be continued untilapproximately 100 nm to 300 nm of the powder is deposited onto the firstregion.

The vapour deposition may continue until the solid layer of theperovskite has a thickness of at least 100 nm. Typically, for instanceit continues until the solid layer of the perovskite has a thickness offrom 100 nm to 100 µm, or for instance from 100 nm to 700 nm.

The inventors have found that a dual source vapour deposition processallows uniform layers of the perovskite to be deposited. Vapourdeposition is one of the most common ways in large manufacturing todeposit thin films of controlled thickness and conventionally refers todeposition of thin films by the condensation of a vaporised form of thedesired film material onto a surface in vacuum. Deposition methods ofinorganic perovskite have been well researched, such as pulsed-laserdeposition and chemical solution deposition. The hybridinorganic-organic perovskite, such as (C₆H₅C₂H₄NH₃)₂Pbl₄ or(C₆H₅C₂H₄NH₃)₂PbBr₄, have been successfully evaporated through singlesource thermal deposition. However, since the deposition methods of thehybrid inorganic-organic perovskite were rarely mentioned because of thesignificant difference in physical and chemical properties between theinorganic and organic materials, the dual-source thermal deposition wasapplied to evaporate the organic source and inorganic sourcesimultaneously but with independent control (V. K. Dwivedi, J. J.Baumberg, and G. V. Prakash, “Direct deposition of inorganic-organichybrid semiconductors and their template-assisted microstructures,”Materials Chemistry and Physics, vol. 137, no. 3, pp. 941-946, January2013). Recently, template-assisted electrochemical deposition wassuggested to obtain a new class of a hybrid perovskite (C₁₂H₂₅NH₃)₂PbI₄in multiple quantum well structures with strong exciton emission. It hasalso been suggested that these materials can be directly carved into 2Dphotonic structures that could be very useful in photovoltaic devices.The deposition of hybrid organic-inorganic perovskite materials isalways challenging, because most organic materials are very volatile anddecompose easily, and this makes control of the deposition process morecomplicated.

In one embodiment, the step (b) of disposing the second region on thefirst region comprises:

producing a solid layer of the perovskite by vapour deposition, whereinthe vapour deposition is a dual source vapour deposition.

The term “dual source vapour deposition”, as used herein, refers to avapour deposition process in which the vapour which is deposited on asubstrate comprises two or more components which originate from twodistinct sources. Typically a first source will produce a vapourcomprising a first component and a second source will produce a vapourcomprising a second component. Dual source vapour deposition may also beextended to include three and four source vapour deposition, althoughdual source deposition is normally preferable.

In one embodiment, the step (b) of disposing the second region on thefirst region comprises:

-   (i) exposing the first region to vapour, which vapour comprises two    reactants for producing said perovskite; and-   (ii) allowing deposition of the vapour onto the first region, to    produce a solid layer of said perovskite thereon;

wherein (i) further comprises producing said vapour comprising tworeactants for producing said perovskite by evaporating a first reactantfrom a first source and evaporating a second reactant from a secondsource.

The reactants may be as defined herein for the production of aperovskite. The vapour may alternatively comprise three or morereactants. The two sources are typically placed at the same distancefrom the first region, often from 10 to 40 cm.

Often the first reactant comprises a first compound comprising (i) afirst cation and (ii) a first anion; and the second reactant comprises asecond compound comprising (i) a second cation and (ii) a second anion.In some cases, the first cation here will be a metal cation. In somecases the second cation here will be an organic cation. Accordingly, thefirst reactant may comprise a first compound comprising (i) a metalcation and (ii) a first anion; and the second reactant may comprise asecond compound comprising (i) an organic cation and (ii) a secondanion. Preferably the metal cation is a divalent metal cation. Forinstance, the metal cation may be a cation selected from Ca²⁺, Sr2⁺,Cd²⁺, Cu2⁺, Ni²⁺, Mn²⁺, Fe²⁺, Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ andEu²⁺. Of these cations, it is preferable that the divalent metal cationis Pb²⁺ or Sn²⁺.

Often the organic cation has the formula (R₁R₂R₃R₄N)+, wherein:

-   R₁ is hydrogen, or unsubstituted or substituted C₁-C₂₀ alkyl, or    unsubstituted or substituted aryl;-   R₂ is hydrogen, or unsubstituted or substituted C₁-C₂₀ alkyl, or    unsubstituted or substituted aryl;-   R₃ is hydrogen, or unsubstituted or substituted C₁-C₂₀ alkyl, or    unsubstituted or substituted aryl; and-   R₄ is hydrogen, or unsubstituted or substituted C₁-C₂₀ alkyl, or    unsubstituted or substituted aryl.

The organic cation may be as defined anywhere herein. Often, the organiccation has the formula (R₅NH₃)+, wherein: R₅ is hydrogen, orunsubstituted or substituted C₁-C₂₀ alkyl. For instance, the organiccation has the formula (R₅NH₃)⁺, wherein: R₅ is methyl, ethyl, propyl orbutyl, preferably methyl or ethyl. In some cases the organic cation maybe a methylammonium cation.

Alternatively, the organic cation has the formula (R₅R₆N═CH—NR₇R₈)⁺,wherein: R₅ is hydrogen, unsubstituted or substituted C₁-C₂₀ alkyl, orunsubstituted or substituted aryl; R₆ is hydrogen, unsubstituted orsubstituted C₁-C₂₀ alkyl, or unsubstituted or substituted aryl; R₇ ishydrogen, unsubstituted or substituted C₁-C₂₀ alkyl, or unsubstituted orsubstituted aryl; and R₈ is hydrogen, unsubstituted or substitutedC₁-C₂₀ alkyl, or unsubstituted or substituted aryl.

Typically, R₅ in the cation (R₅R₆N═CH—NR₇R₈)⁺ is hydrogen, methyl orethyl, R₆ is hydrogen, methyl or ethyl, R₇ is hydrogen, methyl or ethyl,and R₈ is hydrogen, methyl or ethyl. For instance R₅ may be hydrogen ormethyl, R₆ may be hydrogen or methyl, R₇ may be hydrogen or methyl, andR₈ may be hydrogen or methyl.

The organic cation may, for example, have the formula (H₂N═CH—NH₂)⁺

The first and second anions may be any anions, but are typicallyselected from halide ions (e.g. fluoride, chloride, bromide and iodide)or chalcogenide ions (e.g. sulfide, selenide and telluride). Often, theperovskite will be a mixed halide or mixed chalcogenide perovskite andthe first and second anions are different anions selected from halideions or chalcogenide ions. Preferably the first and second anions arehalide anions. Typically, the first and second anions are differentanions selected from halide anions. For instance, the first anion andsecond anion may be one of the following pairs as (first anion:secondanion): (fluoride: chloride), (chloride: fluoride), (fluoride:bromide),(bromide:fluoride), (fluoride:iodide), (iodide: fluoride),(chloride:bromide), (bromide:chloride), (chloride:iodide),(iodide:chloride), (bromide:iodide) or (iodide:bromide).

In some embodiments, the first reactant will comprise a metal dihalide,and the second reactant will comprise a halide salt of an organic acid.For instance, the first reactant may comprise a first compound which isBX₂ and the second reactant may comprise a second compound which is AX’,wherein B is a first cation, X is a first anion, A is a second cationand X′ is a second anion. Each of the cations and anions may be asdefined above. Occasionally, the first reactant comprises a firstcompound which is BX₂ and the second reactant comprises a secondcompound which is AX’, wherein

-   B is a cation selected from Ca²⁺, Sr2⁺, Cd²⁺, Cu²⁺, Ni²⁺, Mn²⁺,    Fe²⁺, Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ and Eu²⁺,-   X is an anion selected from F⁻, Cl⁻, Br⁻ and I⁻,-   A is a cation of formula (R₅NH₃)⁺, wherein: R₅ is hydrogen, or    unsubstituted or substituted C₁-C₂₀ alkyl,-   X′ is an anion selected from F⁻, Cl⁻, Br⁻ and I⁻, and-   X and X′ are different anions.

The first reactant may comprise lead halide or tin halide and the secondreactant may comprise methylammonium halide or ethyl ammonium halide,wherein the halide ion in the first reactant and the second reactant aredifferent. Often, the first reactant comprises tin fluoride and thesecond reactant comprises methylammonium chloride, methylammoniumbromide or methylammonium iodide;

-   the first reactant comprises lead chloride or tin chloride and the    second reactant comprises methylammonium bromide or methylammonium    iodide;-   the first reactant comprises lead bromide or tin bromide and the    second reactant comprises methylammonium chloride or methylammonium    iodide; or-   the first reactant comprises lead iodide or tin bromide and the    second reactant comprises methylammonium chloride or methyl ammonium    bromide.

Preferably, the first reactant comprises lead chloride and the secondreactant comprises methylammonium iodide.

These pairs of compound also apply for other deposition methods of aperovskite, e.g. solution deposition.

Alternatively, A may be an inorganic monovalent cation. For instance, Amay be a cation of a group 1 metal such as Cs⁺. If A is inorganic, thetwo halide anions in each reactant may be the same or different. Forexample, the first reactant may comprise a first compound BX₂, and thesecond reactant may comprise a second compound which is AX, wherein

-   B is a cation selected from Ca²⁺, Sr²⁺, Cd2⁺, Cu²⁺, Ni²⁺, Mn²⁺,    Fe²⁺, Co²⁺, Pd²+, Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ and Eu²⁺,-   X is an anion selected from F⁻, Cl⁻, Br⁻ and I⁻,-   A is Cs⁺,-   X′ is an anion selected from F⁻, Cl⁻, Br⁻ and I⁻, and-   X and X′ are the same or different.

Dual vapour deposition using these reactants can produce layers ofperovskite of formula (IB) as defined above, for instance CsSnBr₃.Alternatively, CsSnBr_(3-y)I_(y) (where y is as defined in formula IIabove) may be produced.

Dual vapour deposition allows the evaporation rate (here given inangstroms per second) of each component to be controlled, thus leadingto a more controlled deposition. Typically, the evaporation rate of thefirst reactant (optionally comprising a metal cation) is from 0.1 to 10Å/s, or from 0.1 to 5 Å/s and the evaporation rate of the secondreactant (optionally comprising an organic cation) is from 1 to 20 Å/sor from 1 to 10 Å/s. The amount of perovskite disposed may be controlledby changing the amount of time for which deposition is effected.Typically, vapour deposition (in either single source or dual sourcecases) may be performed for from 5 to 60 minutes or from 20 to 40minutes. The deposition time will depend on the evaporation rate used.Often, an excess of the second component is preferable, and the molarratio of the first reactant and the second reactant deposited may befrom 1: 1 to 1:16, or from 1:4 to 1:16. Vapour deposition may be stoppedwhen the desired layer thickness is obtained.

Vapour deposition is generally performed in a chamber with a pressure ofless than 10⁻⁴ mbar, for instance less than 10⁻⁵ mbar. The step ofdisposing the second region on the first region by vapour depositionusually further comprises: (iii) heating the solid layer of theperovskite thus produced.

The step of heating the solid layer of the perovskite usually comprisesheating the solid layer of the perovskite in an inert atmosphere.Typically, the temperature at which the solid layer of the perovskite isheated does not exceed 150° C. Thus, the solid layer of the perovskitemay be heated at a temperature of from 30° C. to 150° C., and ispreferably heated at a temperature of from 40° C. to 110° C. The solidlayer of the perovskite may be heated at said temperature until it hasthe desired semiconducting properties. Usually, the solid layer of theperovskite is heated for at least 30 minutes, preferably for at least 1hour. In some embodiments, the solid layer of the perovskite is heateduntil the desired semiconducting properties are obtained, which can bemeasured by routine methods for measuring conductivity and resistivity.The solid layer of the perovskite is in some cases heated until a colourchange is observed, which colour change indicates that the desiredsemiconducting properties are obtained. In the case of the CH₃NH₃PbI₂Clperovskite, the colour change is typically from yellow to brown.

The second region may be disposed on the first region by a methodcomprising disposing a solid layer of a first compound (a firstperovskite precursor) on the first region, and then treating thedisposed layer with a solution of a second compound (a second perovskiteprecursor). This may be referred to as the “two step method”. The solidlayer of a first perovskite precursor may be disposed by vacuumdeposition. This solid layer is then treated with a solution of a secondperovskite precursor. The second precursor in the solution then reactswith the existing solid layer of the first perovskite precursor toproduce a solid layer of the perovskite. The solid layer of a firstperovskite precursor solution may be treated with a solution comprisingthe second perovskite precursor, for instance by dipping the solid layerof a first perovskite precursor in a solution comprising the secondperovskite precursor. The solid layer of a first perovskite precursormay also be treated by disposing the solution comprising the secondperovskite precursor on the solid layer of the first perovskiteprecursor.

The first perovskite precursor is a first compound comprising (i) afirst cation and (ii) a first anion and the second perovskite precursoris a second compound comprising (i) a second cation and (ii) a secondanion. The first and second cations are usually as defined herein forthe perovskite, and the first and second anions may be the same ordifferent and may be as defined herein for the first and second anions.

In one embodiment, the step of (b) disposing the second region on thefirst region comprises:

-   (i) exposing the first region to vapour, which vapour comprises a    first perovskite precursor compound, and allowing deposition of the    vapour onto the first region, to produce a solid layer of the first    perovskite precursor compound thereon; and-   (ii) treating the resulting solid layer of the first perovskite    precursor compound with a solution comprising a second perovskite    precursor compound, and thereby reacting the first and second    perovskite precursor compounds to produce said layer of the    perovskite semiconductor without open porosity,

wherein

-   the first perovskite precursor compound comprises (i) a first cation    and (ii) a first anion and the second perovskite precursor compound    comprises (i) a second cation and (ii) a second anion.

The first cation, first anion, second cation and second anion may be asdescribed anywhere herein for the perovskite.

In some cases, the first cation here will be a metal cation. In somecases the second cation here will be an organic cation. Accordingly, thefirst compound may comprise (i) a metal cation and (ii) a first anion;and the second compound may comprise (i) an organic cation and (ii) asecond anion. Preferably the metal cation is a divalent metal cation.For instance, the metal cation may be a cation selected from Ca²⁺, Sr2⁺,Cd²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Fe²⁺, Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ andEu²⁺. Of these cations, it is preferable that the divalent metal cationis Pb²⁺ or Sn²⁺.

The first and second anions, which may be the same or different, may beany anions, but are typically selected from halide ions (e.g. fluoride,chloride, bromide and iodide) or chalcogenide ions (e.g. sulfide,selenide and telluride).

Often, the perovskite produced will be a mixed halide or mixedchalcogenide perovskite and the first and second anions are differentanions selected from halide ions or chalcogenide ions.

Preferably the first and second anions are halide anions. Typically, thefirst and second anions are different anions selected from halideanions. For instance, the first anion and second anion may be one of thefollowing pairs as (first anion:second anion): (fluoride:chloride),(chloride: fluoride), (fluoride:bromide), (bromide:fluoride),(fluoride:iodide), (iodide: fluoride), (chloride:bromide),(bromide:chloride), (chloride:iodide), (iodide:chloride),(bromide:iodide) or (iodide:bromide).

The organic cation may be selected from (R₁R₂R₃R₄N)⁺, (R₅NH₃)⁺, or(R₅R₆N═CH—NR₇R₈)⁺ wherein R₁ to R₈ may be as defined above.

Often, the first compound has the formula BX₂ and the second compoundhas the formula AX’, wherein

-   B is a cation selected from Ca²⁺, Sr²⁺, Cd2⁺, Cu²⁺, Ni²⁺, Mn²⁺,    Fe²⁺, Co²⁺, Pd²⁺, Ge²⁺,Sn²⁺, Pb²⁺, Yb²⁺ and Eu²⁺,-   X is an anion selected from F⁻, Cl⁻, Br⁻ and I⁻,-   A is a cation of formula (R₅NH₃)⁺, wherein: R₅ is hydrogen, or    unsubstituted or substituted C₁-C₂₀ alkyl,-   X′ is an anion selected from F⁻, Cl⁻, Br⁻ and I⁻, and-   X and X′ are the same or different anions.

Often, the first perovskite precursor compound may be selected from leadfluoride, lead chloride, lead bromide, lead iodide, tin fluoride, tinchloride, tin bromide, or tin iodide. Typically, it is lead chloride orlead iodide. Often, the second perovskite precursor compound is selectedfrom methylammonium fluoride, methylammonium chloride, methylammoniumbromide, methylammonium iodide, ethylammonium fluoride, ethylammoniumchloride, ethylammonium bromide, or ethylammonium iodide. Typically, itis methylammonium iodide.

Typically, the vapour deposition of the first perovskite precursorcompound is allowed to continue until the solid layer of the firstcompound has a desired thickness, for instance a thickness of from 10 nmto 100 µm, or more typically from 10 nm to 10 µm. Preferably, the vapourdeposition is allowed to continue until the solid layer of the firstcompound has a thickness of from 50 nm to 1000 nm, or for instance from100 nm to 700 nm. For instance, deposition may be continued untilapproximately 100 nm to 300 nm of the first compound is deposited ontothe first region.

The vapour deposition may continue until the solid layer of the firstperovskite precursor compound has a thickness of from 100 nm to 100 µmor from 100 nm to 700 nm.

The rate of evaporation of the first compound may be from 0.1 to 10 Å/s,or from 1 to 5 Å/s. The vapour deposition is generally performed in achamber with a pressure of less than 10⁻⁴ mbar, for instance less than10⁻⁵ mbar. The temperature at which the first compound is evaporated maybe from 200° C. to 500° C., or from 250° C. to 350° C.

Typically, step (iii) of exposing the resulting solid layer of the firstcompound to a solution comprising a second compound to allow theformation of the second region comprises dipping the substratecomprising the solid layer of the first compound in the solutioncomprising the second compound for a sufficient time to form the secondregion, i.e. the layer of semiconducting perovskite without openporosity. Step (iii) may comprise dipping the substrate comprising thesolid layer of the first compound in the solution comprising the secondcompound for from 1 to 60 minutes, or from 5 to 15 minutes. Dipping thesubstrate comprising the solid layer of the first compound in thesolution comprising the second compound may be referred to asdip-coating.

The solution comprising the second perovskite precursor compoundcomprises a solvent and the second compound. The solvent may be anysolvent defined herein. The solvent may be dimethylformamide, ethanol orisopropanol. The solvent may be isopropanol. The concentration of thesecond compound in the solvent may be from 5 to 50 mg/ml or from 10 to30 mg/ml.

After exposing the resulting solid layer of the first perovskiteprecursor compound to a solution comprising a second compound to allowformation of the second region (for instance by dip coating), thesubstrate may be annealed. For instance, the substrate may be heated atfrom 80° C. to 200° C. or from 100° C. to 150° C. The substrates may beheated for from 1 to 60 minutes, or from 5 to 15 minutes. The substratesmay be annealed in a nitrogen atmosphere.

Solution deposition methods may be used to dispose the second region onthe first region. Thus, in some embodiments, the step of (b) disposingthe second region on the first region comprises:

-   (i) disposing one or more precursor solutions on the first region,    which one or more precursor solutions comprise: said perovskite    dissolved in a solvent, or one or more reactants for producing said    perovskite dissolved in one or more solvents; and-   (ii) removing the one or more solvents to produce on the first    region a solid layer of the perovskite.

Again, the perovskite may be any of the perovskites discussedhereinbefore for the optoelectronic device of the invention, and istypically a perovskite of formula (I), (IA) or (II) as definedhereinbefore.

Also, the one or more reactants for producing said perovskite maycomprise the reactant types discussed above for the process forsynthesising the perovskite compounds.

Thus, the one or more reactants may comprise:

-   (a) a first compound comprising (i) a first cation and (ii) a first    anion; and-   (b) a second compound comprising (i) a second cation and (ii) a    second anion,

as defined hereinbefore for the process for producing the perovskitecompounds employed in the optoelectronic device of the invention.

More particularly, the one or more reactants may comprise:

-   (a) a first compound comprising (i) a metal cation and (ii) a first    anion; with-   (b) a second compound comprising (i) an organic cation and (ii) a    second anion; wherein the first and second anions are different    anions selected from halide anions or chalcogenide anions, as    defined hereinbefore for the process for producing the perovskite    compounds employed in the optoelectronic device of the invention.    The organic cation may be as defined hereinbefore for the process    for producing the perovskite.

For instance, the one or more reactants may comprise:

-   (a) a first compound comprising (i) a metal cation and (ii) a first    halide anion; with-   (b) a second compound comprising (i) an organic cation and (ii) a    second halide anion; wherein the first and second halide anions are    different halide anions, as defined hereinbefore for the process for    producing the perovskite compounds employed in the optoelectronic    device of the invention.

For instance, when the perovskite being deposited is CH₃NH₃PbI₂Cl, theone or more reactants typically comprise (a) PbI₂, and (b) CH₃NH₃Cl.

Typically, the step of (b) disposing the second region on the firstregion comprises:

-   (i) disposing a precursor solution on the first region, which    precursor solution comprises said perovskite dissolved in a solvent;    and-   (ii) removing the solvent to produce on the first region a solid    layer of the perovskite.

The perovskite may be any of the perovskites discussed hereinbefore forthe optoelectronic device of the invention, and is typically aperovskite of formula (I), (IA) or (II) as defined hereinbefore.

Usually, the steps of (i) disposing a precursor solution on the firstregion, and (ii) removing the solvent, comprise spin coating orslot-dye-coating the precursor solution or solutions onto the firstregion, to produce on the first region said solid layer of theperovskite. Typically, said coating is carried out in an inertatmosphere, for instance under nitrogen. The spin coating is usuallyperformed at a speed of from 1000 to 2000 rpm. The spin coating istypically carried out for 30 seconds to 2 minutes.

The precursor solution or solutions may be disposed by spin coating ontothe first region to produce on the first region said solid layer of theperovskite.

The steps of disposing the precursor solution or solutions on the firstregion and removing the solvent or solvents are carried out until thesolid layer of the perovskite has a desired thickness, for instance athickness of from 10 nm to 100 µm, more typically from 10 nm to 10 µm.For instance, the steps of disposing the precursor solution or solutionson the first region and removing the solvent or solvents may be carriedout until the solid layer of the perovskite has a thickness of from 50nm to 1000 nm, or for instance from 100 nm to 700 nm.

The steps of disposing the precursor solution or solutions on the firstregion and removing the solvent or solvents may be carried out until thesolid layer of the perovskite has a thickness of from 100 nm to 100 µm,or from 100 nm to 700 nm.

The step of disposing the second region on the first region (by solutiondeposition) usually further comprises: (iii) heating the solid layer ofthe perovskite thus produced.

The step of heating the solid layer of the perovskite usually comprisesheating the solid layer of the perovskite in an inert atmosphere.Typically, the temperature at which the solid layer of the perovskite isheated does not exceed 150° C. Thus, the solid layer of the perovskitemay be heated at a temperature of from 30° C. to 150° C., and ispreferably heated at a temperature of from 40° C. to 110° C. The solidlayer of the perovskite may be heated at said temperature until it hasthe desired semiconducting properties. Usually, the solid layer of theperovskite is heated for at least 30 minutes, preferably for at least 1hour. In some embodiments, the solid layer of the perovskite is heateduntil the desired semiconducting properties are obtained, which can bemeasured by routine methods for measuring conductivity and resistivity.The solid layer of the perovskite is in some cases heated until a colourchange is observed, which colour change indicates that the desiredsemiconducting properties are obtained. In the case of the CH₃NH₃PbI₂Clperovskite, the colour change is typically from yellow to brown.

In some embodiments of the process of the invention (e.g. when thephotoactive region of the device being produced has no scaffoldmaterial), the second region consists of said layer of said perovskitesemiconductor without open porosity.

In another embodiment of the process of the invention, however, saidphotoactive region comprises:

-   said n-type region;-   said p-type region; and, disposed between the n-type region and the    p-type region:    -   (i) a first layer which comprises a scaffold material and a        perovskite semiconductor; and    -   (ii) a capping layer disposed on said first layer, which capping        layer is said layer of a perovskite semiconductor without open        porosity,-   wherein the perovskite semiconductor in the capping layer is in    contact with the perovskite semiconductor in the first layer,-   and the process comprises:    -   (a) providing said first region;    -   (b) disposing said second region on the first region, wherein        the second region comprises:        -   (i) a first layer which comprises a scaffold material and a            perovskite semiconductor; and        -   (ii) a capping layer on said first layer, which capping            layer is said layer of a perovskite semiconductor without            open porosity, wherein the perovskite semiconductor in the            capping layer is in contact with the perovskite            semiconductor in the first layer; and    -   (c) disposing said third region on the second region.

Generally, the scaffold material is porous and said first layercomprises said perovskite semiconductor disposed in pores of thescaffold material. Thus, typically, in this embodiment, the step of (b)disposing said second region on the first region comprises:

-   (i) disposing a scaffold material on the first region; and-   (ii) disposing said perovskite into pores of the scaffold material    in order to produce said first layer and further disposing said    perovskite onto the first layer to produce said capping layer.    Usually, the “disposing” of said perovskite into pores of the    scaffold material and the “further disposing” of said perovskite    onto the first layer are performed together in a single step, for    instance by solution deposition step or by vapour deposition. They    are typically performed by solution deposition.

Typically, the step (i) of disposing a scaffold material on the firstregion comprises:

-   disposing a scaffold composition onto the first region, which    scaffold composition comprises the scaffold material, one or more    solvents, and optionally a binder; and-   removing the one or more solvents and, when present, the binder.

The binder is typically a polymer binder, such as, for instance, ethylcellulose.

This step typically comprises screen printing, doctor blading,slot-dye-coating or spin coating the scaffold composition onto the firstregion.

The films are typically subsequently heated, either to a temperature ofaround 500° C. (and usually held there for around 30 minutes) in orderto degrade and remove any polymer binder that is present (hightemperature sintering), or, in the absence of binder, they weretypically heated around 120° C. and held there for around 90 minutes(low temperature sintering). The substrates were typically then cooledready for perovskite solution deposition.

Thus, usually, the step (i) of disposing the scaffold material on thefirst region further comprises heating the scaffold composition.

Of importance to low-temperature processing of the mesoporous scaffoldlayer is the absence of a thermo-degradable polymer binder in thenanoparticle paste during deposition. Instead, nanoparticles aredeposited from a colloidal dispersion in one or more solvents. Atlow-temperatures, adhesion between the particles and to the substrate isthought to proceed by dehydration of surface hydroxide groups [T.Miyasaka et al., Journal of the Electrochemical Society, vol. 154, p.A455, 2007]. The inventors also show that the porosity can be tuned bymixing two solvents in the dispersion with different viscosities andboiling points.

Thus, in a preferred embodiment, the scaffold composition does notcomprise a binder and the temperature at which the scaffold compositionis heated does not exceed 150° C.

Thus, typically, the step (i) of disposing a scaffold material on thefirst region comprises:

-   disposing a scaffold composition onto the first region, which    scaffold composition comprises the scaffold material and one or more    solvents; and-   removing the one or more solvents.

Usually, the step (i) of disposing the scaffold material on the firstregion further comprises heating the scaffold composition to atemperature that does not exceed 150° C. Typically, the scaffoldcomposition is heated to a temperature of from 60° C. to 150° C. Thescaffold composition is heated to said temperature for a suitable time,for instance until all solvents are removed. Typically, the scaffoldcomposition is heated to said temperature for at least 30 minutes, moretypically for at least 1 hour, or for at least 90 minutes.

Usually, the step (i) of disposing the scaffold material on the firstregion is performed until the scaffold material which is disposed on thefirst region has a desired thickness, for instance a thickness of from 5nm to 500 nm, preferably from 30 nm to 200 nm.

The scaffold material employed in the scaffold composition may be asdefined above for the optoelectronic device of the invention. Often, thescaffold material employed is titania or alumina.

The one or more solvents employed in the scaffold composition maycomprise a mixture of two or more solvents with different viscositiesand boiling points, for instance a mixture of two solvents withdifferent viscosities and boiling points. The use of two or moresolvents with different viscosities and boiling points is advantageous,as the inventors have shown that the porosity of the scaffold materialdisposed on the first region can be tuned by varying the ratio of thetwo or more solvents. The two or more solvents may for instance comprisetwo or more different alcohols, for instance two different alcohols.Thus, for example, the two or more solvents may comprise two solventsselected from ethanol, propanol, butanol and terpineol, or from ethanol,iso-propanol, tert-butanol and terpineol.

Typically, step (ii), of disposing said perovskite into pores of thescaffold material in order to produce said first layer and furtherdisposing said perovskite onto the first layer to produce said cappinglayer is carried out until the capping layer has a desired thickness,for instance a thickness of from 10 nm to 100 µm, or more typically athickness of from 10 nm to 10 µm, preferably from 50 nm to 1000 nm, orfor instance a thickness of from 100 nm to 700 nm.

Solution deposition methods may be used to dispose said perovskite intopores of the scaffold material in order to produce said first layer andfurther dispose said perovskite onto the first layer to produce saidcapping layer. Thus, in some embodiments, the step (ii), of disposingsaid perovskite into pores of the scaffold material in order to producesaid first layer and further disposing said perovskite onto the firstlayer to produce said capping layer, comprises:

-   disposing one or more precursor solutions onto the scaffold    material, which one or more precursor solutions comprise: said    perovskite dissolved in a solvent, or one or more reactants for    producing said perovskite dissolved in one or more solvents; and-   removing the one or more solvents to produce solid perovskite in    pores of the scaffold material and a solid capping layer of the    perovskite disposed on the first layer.

The perovskite may be any of the perovskites discussed hereinbefore forthe optoelectronic device of the invention, and is typically aperovskite of formula (I), (IA) or (II) as defined hereinbefore.

Also, the one or more reactants for producing said perovskite maycomprise the reactant types discussed above for the process forsynthesising the perovskite compounds.

Thus, the one or more reactants may comprise:

-   (a) a first compound comprising (i) a first cation and (ii) a first    anion; and-   (b) a second compound comprising (i) a second cation and (ii) a    second anion,

as defined hereinbefore for the process for producing the perovskitecompounds employed in the optoelectronic device of the invention.

More particularly, the one or more reactants may comprise:

-   (a) a first compound comprising (i) a metal cation and (ii) a first    anion; with-   (b) a second compound comprising (i) an organic cation and (ii) a    second anion; wherein the first and second anions are different    anions selected from halide anions or chalcogenide anions, as    defined hereinbefore for the process for producing the perovskite    compounds employed in the optoelectronic device of the invention.

For instance, the one or more reactants may comprise:

-   (a) a first compound comprising (i) a metal cation and (ii) a first    halide anion; with-   (b) a second compound comprising (i) an organic cation and (ii) a    second halide anion; wherein the first and second halide anions are    different halide anions,

as defined hereinbefore for the process for producing the perovskitecompounds employed in the optoelectronic device of the invention.

For instance, when the perovskite being deposited is CH₃NH₃PbI₂Cl, theone or more reactants typically comprise (a) PbI₂, and (b) CH₃NH₃Cl.

Typically, the step (ii), of disposing said perovskite into pores of thescaffold material in order to produce said first layer and furtherdisposing said perovskite onto the first layer to produce said cappinglayer, comprises:

-   disposing a precursor solution onto the scaffold material, which    precursor solution comprises said perovskite dissolved in a solvent;    and-   removing the solvent to produce solid perovskite in pores of the    scaffold material and a solid capping layer of the perovskite    disposed on the first layer.

The perovskite may be any of the perovskites discussed hereinbefore forthe optoelectronic device of the invention, and is typically aperovskite of formula (I), (IA) or (II) as defined hereinbefore.

Usually, the steps of disposing a precursor solution onto the scaffoldmaterial, and removing the solvent or solvents, comprise spin coating orslot-dye-coating the precursor solution or solutions onto the scaffoldmaterial, to produce said solid perovskite in pores of the scaffoldmaterial and said solid capping layer of the perovskite disposed on thefirst layer. Typically, the coating is carried out in an inertatmosphere, for instance under nitrogen. The spin coating may forinstance be performed at a speed of from 1000 to 2000 rpm. The spincoating is typically carried out for 30 seconds to 2 minutes.

The steps of disposing the precursor solution or solutions onto thescaffold material and removing the solvent or solvents are carried outuntil the solid capping layer of the perovskite has a desired thickness,for instance a thickness of from 10 nm to 100 µm, or more typically from10 nm to 10 µm, or, for instance, from 50 nm to 1000 nm, preferably from100 nm to 700 nm.

Usually, the step of (b) disposing said second region on the firstregion further comprises: (iii) heating the perovskite.

The step of heating the perovskite usually comprises heating theperovskite in an inert atmosphere, for instance under nitrogen.Typically, the temperature at which the perovskite is heated does notexceed 150° C. Thus, the perovskite may be heated at a temperature offrom 30° C. to 150° C., and is preferably heated at a temperature offrom 40° C. to 110° C. The perovskite may be heated at said temperatureuntil it has the desired semiconducting properties. Usually, theperovskite is heated for at least 30 minutes, preferably for at least 1hour. In some embodiments, the perovskite is heated until the desiredsemiconducting properties are obtained, which can be measured by routinemethods for measuring conductivity and resistivity. The perovskite is insome cases heated until a colour change is observed, which colour changeindicates that the desired semiconducting properties have been obtained.In the case of the CH₃NH₃PbI₂Cl perovskite, the colour change istypically from yellow to brown.

Usually, in the process of the invention for producing an optoelectronicdevice the first region is disposed on a first electrode. That is tosay, the first region is usually already disposed on a first electrode.

The process of the invention for producing an optoelectronic device mayhowever further comprise a step of:

disposing the first region on a first electrode.

This step is generally carried out before the step of disposing thesecond region on the first region.

The first and second electrodes are an anode and a cathode, one or bothof which is transparent to allow the ingress of light. The choice of thefirst and second electrodes may depend on the structure type.

Typically, first electrode onto which the second region is disposed istin oxide, more typically fluorine-doped tin oxide (FTO), which isusually a transparent or semi-transparent material. Thus, the firstelectrode is usually transparent or semi-transparent and typicallycomprises FTO. Usually, the thickness of the first electrode is from 200nm to 600 nm, more usually, from 300 to 500 nm. For example thethickness may be 400 nm. Typically, the FTO is coated onto a glasssheet. Often, the FTO coated glass sheets are etched with zinc powderand an acid to produce the required electrode pattern. Usually the acidis HCl. Often the concentration of the HCl is about 2 molar. Typically,the sheets are cleaned and then usually treated under oxygen plasma toremove any organic residues. Usually, the treatment under oxygen plasmais for less than or equal to 1 hour, typically about 5 minutes. Thefirst and second electrodes may be as described anywhere hereinbefore,for instance the first electrode may comprise FTO, ITO or AZO.

The steps of disposing the first region on a first electrode anddisposing the third region on the second region, comprise deposition ofthe p-type and the n-type regions, i.e. deposition of the one or morep-type layers and deposition of the one or more n-type layers. Thep-type and the n-type regions, and the one or more p-type layers and theone or more n-type layers, may be as further defined hereinbefore.

The step of depositing a layer of a p-type or n-type inorganic compoundmay, for instance, comprise depositing the layer by spin coating or byslot-dye-coating the compound or a precursor thereof, or by spraypyrolysis. For instance, a compact layer of titania may be produced byspincoating a (mildly) acidic titanium-isopropoxide sol in a suitablesolvent, such as ethanol. Such a sol can be prepared by mixing titaniumisopropoxide and anhydrous ethanol with a solution of HCl in anhydrousethanol. After spincoating, the layer is typically dried at atemperature not exceeding 150° C. Optionally, the compact layer wassubsequently heated to 500° C. for 30 minutes on a hotplate in air.Alternatively, such a compact layer may be produced by, spray pyrolysisdeposition. This will typically comprise deposition of a solutioncomprising titanium diisopropoxide bis(acetylacetonate), usually at atemperature of from 200 to 300° C., often at a temperature of about 250°C. Usually the solution comprises titanium diisopropoxidebis(acetylacetonate) and ethanol, typically in a ratio of from 1:5 to1:20, more typically in a ratio of about 1:10.

Such methods can be applied to other p-type or n-type inorganicmaterials, to produce n-type and p-type layers in the optoelectronicdevices of the invention.

Deposition of an organic, molecular or polymeric hole transporter orelectron transporter material my be achieved by spin-coating a solutionof the material in a suitable solvent. The p-type hole transporter,spiro-OMeTAD, for instance, is typically dissolved in chlorobenzene.Usually the concentration of spiro-OMeTAD in chlorobenzene is from 150to 225 mg/ml, more usually the concentration is about 180 mg/ml. Anadditive may be added to the hole transporter or electron transportermaterial. The additive may be, for instance, tBP, Li-TFSi, an ionicliquid or an ionic liquid with a mixed halide(s).

The process of the invention for producing an optoelectronic device mayfurther comprise: (d) disposing a second electrode on the third region.

Usually, the second electrode comprises a high work function metal, forinstance gold, silver, nickel, palladium or platinum, and typicallysilver. Usually, the thickness of the second electrode is from 50 nm to250 nm, more usually from 100 nm to 200 nm. For example, the thicknessof the second electrode may be 150 nm.

The second electrode is typically disposed on the third region by vapourdeposition. Often, the step of producing a second electrode comprisesplacing a film comprising the hole transporting material in a thermalevaporator. Usually, the step of producing a second electrode comprisesdeposition of the second electrode through a shadow mask under a highvacuum. Typically, the vacuum is about 10⁻⁶ mBar.

The second electrode may, for example, be an electrode of a thicknessfrom 100 to 200 nm. Typically, the second electrode is an electrode of athickness from 150 nm.

Alternatively, the process of the invention for producing anoptoelectronic device may be a process for producing an invertedoptoelectronic device.

Accordingly, the invention provides a process for producing an invertedoptoelectronic device comprising a photoactive region, which photoactiveregion comprises:

-   an n-type region comprising at least one n-type layer;-   a p-type region comprising at least one p-type layer; and, disposed    between the n-type region and the p-type region:-   a layer of a perovskite semiconductor without open porosity,

which process comprises:

-   (a) providing a first region;-   (b) disposing a second region on the first region, which second    region comprises a layer of a perovskite semiconductor without open    porosity; and-   (c) disposing a third region on the second region,

wherein:

-   the first region is a p-type region comprising at least one p-type    layer and the third region is an n-type region comprising at least    one n-type layer, and the first region is disposed on a first    electrode.

Typically, the first electrode comprises a transparent orsemi-transparent material. Typically, the first electrode comprises atransparent conducting oxide, for example FTO, ITO or AZO. Preferably,the first electrode comprises FTO. The first electrode may be disposedon a glass substrate.

Each of the steps in the process for producing an invertedoptoelectronic device may be as defined anywhere herein for a processaccording to the invention for producing an optoelectronic device. Eachof the components used or present in the process may be as defined foran optoelectronic device according to the invention.

The first region, which is a p-type region, may be as defined anywhereherein for a p-type region. Often, the first region comprises a layer ofPEDOT:PSS. Crosslinking may be performed to insolubilise the p-typeregion so that it is not partially dissolved during the disposition ofthe second region, if the disposition process might lead to the p-typelayer being dissolved. Occasionally, therefore, the layer of PEDOT:PSScomprises crosslinked PEDOT:PSS. Crosslinking may be performed using alewis acid, for instance a metal cation such as Fe³⁺ or Mg²⁺. Forinstance, (a) may comprise

-   (i) providing a first region comprising a layer of PEDOT:PSS and-   (ii) treating the layer with aqueous FeCl₃ to produce a layer of    PEDOT:PSS comprising crosslinked PEDOT:PSS.

The second region, which is an n-type region, may be as defined anywhereherein for an n-type region. Often, the n-type region comprises acompact layer of an inorganic n-type semiconductor, such as thosedefined herein. Typically, the n-type region comprises a compact layerof titanium dioxide. In some embodiments, the n-type region furthercomprises a layer of [60]PCBM.

Therefore, in some embodiments, (c) comprises

-   (i) disposing on the second region a layer of [60]PCBM; and-   (ii) disposing on the layer of [60]PCBM a compact layer of titanium    dioxide.

In an inverted device, a second electrode may be disposed on the thirdregion which is an n-type region. Accordingly, the process may furthercomprise

(d) disposing a second electrode on the third region.

The second electrode may be disposed directly onto the third region, orthere may be further intervening layers. Typically, the second electrodeis in contact with the third region. The second electrode may be asdefined anywhere herein and typically comprises a metal. For instance,the second electrode may comprise aluminium, gold, silver, nickel,palladium or platinum, and typically aluminium, silver or gold. In oneembodiment, the second electrode comprises silver, gold or aluminium.For example, if the n-type region comprises a compact layer of titaniumand a layer of [60]PCBM, the second electrode may comprise aluminium.The second electrode may be disposed by any technique such as thosedescribed herein, although it is typically disposed by vacuumdeposition. Accordingly, the second electrode may be disposed by vacuumdeposition. Alternatively, the process of the invention for producing anoptoelectronic device may be a process for producing a tandem- or multi-junction optoelectronic device which further comprises:

-   (d) disposing a tunnel junction on the third region;-   (e) disposing a further photoactive region on the tunnel junction,    which is the same as or different from the photoactive region    defined hereinbefore;-   (f) optionally repeating steps (d) and (e); and-   (g) disposing a second electrode on the further photoactive region    disposed in the preceding step.

In a process for producing a tandem or multi-junction device accordingto the invention, the further photoactive region may be as definedanywhere hereinbefore for tandem optoelectronic devices according to theinvention. In particular, the further photoactive region may comprise alayer of crystalline silicon, or may comprises a thin film of CIGS, CISor CZTSSe.

In a preferred embodiment of the process of the invention for producingan optoelectronic device, the entire process is performed at atemperature or temperatures not exceeding 150° C.

In the process of the invention for producing an optoelectronic device,the optoelectronic device may be as further defined hereinbefore for theoptoelectronic device of the invention.

The invention further provides an optoelectronic device which isobtainable by the process of the invention for producing anoptoelectronic device.

The present invention is further illustrated in the Examples whichfollow:

EXAMPLES Experimental Methods For Device Preparation Preparation ofAl₂O₃ Paste With Polymer Binder

An aluminum oxide dispersion was purchased from Sigma-Aldrich (10%wt inwater) and was washed in the following manner: it was centrifuged at7500 rpm for 6h, and redispersed in Absolute Ethanol (Fisher Chemicals)with an ultrasonic probe; which was operated for a total sonication timeof 5 minutes, cycling 2 seconds on, 2 seconds off. This process wasrepeated 3 times.

For every 10 g of the original dispersion (1 g total Al₂O₃) thefollowing was added: 3.33 g of α-terpineol and 5 g of a 50:50 mix ofethyl-cellulose 10 cP and 46 cP purchased from Sigma Aldrich in ethanol,10% by weight. After the addition of each component, the mix was stirredfor 2 minutes and sonicated with the ultrasonic probe for 1 minute ofsonication, using a 2 seconds on 2 seconds off cycle. Finally, theresulting mixture was introduced in a rotary evaporator to remove excessethanol and achieve the required thickness when doctor blading,spin-coating or screen printing.

Preparation of TiO₂ Paste With Polymer Binder

A titanium dioxide dispersion containing a polymer binder (DSL 18NR-T)was purchased from Dyesol. It was diluted at a 3:1 weight ratio ofabsolute ethanol (Fisher Chemicals):DSL 18NR-T with an ultrasonic probe;which was operated for a total sonication time of 5 minutes, cycling 2seconds on, 2 seconds off.

Preparation of Al₂O₃ Paste Without Polymer Binder

An aluminum oxide dispersion was purchased from Sigma-Aldrich (20 % byweight in isopropanol). This was diluted in 16 volume equivalents ofisopropanol.

Preparation of TiO₂ Paste Without Polymer Binder

An titanium dioxide powder (P25) was purchased from (Degussa) anddispersed in ethanol at 20 mg/ml. This was diluted in 16 volumeequivalents of ethanol.

Preparation of Methylammonium Iodide Precursor and Perovskite PrecursorSolution

Methylamine (CH₃NH₂) solution 33 wt. % in absolute ethanol(Sigma-Aldrich) was reacted with hydroiodic acid 57 wt. % in water(Sigma-Aldrich) at 1:1 molar ratio under nitrogen atmosphere inanhydrous ethanol 200 proof (Sigma-Aldrich). Typical quantities were 24ml methylamine, 10 ml hydroiodic acid and 100 ml ethanol.Crystallisation of methylammonium iodide (CHNH₃I) was achieved using arotary evaporator. A white coloured precipitate was formed indicatingsuccessful crystallisation.

The methylamine can be substituted for other amines, such as ethylamine,n-butylamine, tert-butylamine, octylamine etc. in order to alter thesubsequent perovskite properties. In addition, the hydroiodic acid canbe substituted with other acids to form different perovskites, such ashydrochloric acid.

To prepare the precursor solution methylammonium iodide (CHNH₃I)precipitate and lead (II) chloride (Sigma-Aldrich) was dissolved indimethylformamide (C₃H₇NO) (Sigma-Aldrich) at 1:1 molar ratio at 30 vol.%.

Cleaning and Etching of the Substrate and Transparent Electrode

Fluorine doped tin oxide (F:SnO₂/FTO) coated glass sheets (TEC 15, 15Ω/square, Pilkington USA) were etched with zinc powder and HCl (2 M) togive the required electrode pattern. The sheets were subsequentlycleaned with soap (2% Hellmanex in water), deionised water, acetone,ethanol and finally treated under oxygen plasma for 5 minutes to removeany organic residues.

Deposition of the Compact TiO₂ layer

The patterned FTO sheets were then coated with a compact layer of TiO₂by spincoating a mildly acidic titanium-isopropoxide (Sigma-Aldrich) solin ethanol. The sol was prepared by mixing titaniumisopropoxide:anhydrous ethanol at a weight ratio of 0.71:4 with anacidic solution of 2 M HCl:anhydrous ethanol at a weight ratio of0.07:4. After spincoating (speed = 2000 rpm, acceleration = 2000 rpm/s,time = 60 s), the substrates were dried at 150° C. on a hotplate for 10minutes. Optionally, the compact layer was subsequently heated to 500°C. for 30 minutes on a hotplate in air.

Deposition of the Thin Mesoporous Metal Oxide Layer

The insulating metal oxide paste (e.g. the Al₂O₃ paste) was applied ontop of the compact metal oxide layer via screen printing, doctor bladecoating or spin-coating, through a suitable mesh, doctor blade height orspin-speed to create a film with a thickness of ~100 nm. The films weresubsequently either heated to 500° C. and held there for 30 minutes inorder to degrade and remove the polymer binder (high temperaturesintering), or, in the absence of binder, heated to 120° C. and heldthere for 90 minutes (low temperature sintering). The substrates werethen cooled ready for perovskite solution deposition.

Solution Deposition of the Perovskite Precursor and Formation of theSemiconducting Perovskite Thin-Film

40 µl of the perovskite precursor solution in dimethylformamide(methylammonium iodide lead (II) chloride (CH₃NH₃PbCl₂I)) at a volumeconcentration of 30 % was dispensed onto each prepared mesoporouselectrode film and spin-coated at 1500 rpm for 60 s in an inert nitrogenenvironment. The coated films were then placed on a hot plate set at100° C. and left for 60 minutes in nitrogen, prior to cooling. Duringthe drying procedure at 100 degrees, the coated electrode changed colourfrom light yellow to dark brown, indicating the formation of the desiredperovskite film with the semiconducting properties.

Evaporated Deposition of the Perovskite Precursor and Formation of theSemiconducting Perovskite Thin-Film

A 1:1 molar ratio of PbI₂ and CH₃NH₃Cl was ground with a pestle andmortar for 15 minutes to form a bulk perovskite powder. This formed apowder which was desiccated in a nitrogen environment for >12 hours. Acrucible of perovskite powder was transferred to an evaporation chamberwhich was subsequently evacuated. The crucible was slowly heated to 300°C. When the source temperature reached 100° C., a shutter was opened tocommence deposition onto the substrates. The heater was periodicallyswitched off to maintain a pressure of 10⁻⁴ mbar in the chamber.Evaporation continued until a thin film of approximately 100-300 nm hadbeen deposited onto the substrates. Following evaporation the substratewith evaporated material was heated to 50° C. for 1 hour in a nitrogenenvironment.

Preparation of Perovskites Comprising a Formamidinium Cation

As an alternative to ammonium ions, formamidinium cations may be used.Formamidinium iodide (FOI) and formamidinium bromide (FOBr) weresynthesised by reacting a 0.5 M molar solution of formamidinium acetatein ethanol with a 3x molar excess of hydroiodic acid (for FOI) orhydrobromic acid (for FOBr). The acid was added dropwise whilst stirringat room temperature, then left stirring for another 10 minutes. Upondrying at 100° C., a yellow-white powder is formed, which is then driedovernight in a vacuum oven before use. To form FOPbI3 and FOPbBr₃precursor solutions, FOI and PbI₂ or FOBr and PbBr₂ were dissolved inanhydrous N,N-dimethylformamide in a 1:1 molar ratio, 0.88 millimoles ofeach per ml, to give 0.88 M perovskite precursor solutions. To form theFOPbI_(3z)Br_(3(1-z)) perovskite precursors, mixtures were made of theFOPbI3 and FOPbBr₃ 0.88 M solutions in the required ratios, where zranges from 0 to 1.

Films for characterisation or device fabrication were spin-coated in anitrogen-filled glovebox, and annealed at 170° C. for 25 minutes in thenitrogen atmosphere.

Hole-transporter Deposition and Device Assembly

The hole transporting material used was2,2(,7,7(-tetrakis-(N,N-di-methoxyphenylamine)9,9(-spirobifluorene))(spiro-OMeTAD, Lumtec, Taiwan), which was dissolved in chlorobenzene ata typical concentration of 180 mg/ml. Tertbutyl pyridine (tBP) was addeddirectly to the solution with a volume to mass ratio of 1:26 µl/mgtBP:spiro-MeOTAD. Lithium bis(trifluoromethylsulfonyl)amine salt(Li-TFSI) ionic dopant was pre-dissolved in acetonitrile at 170 mg/ml,then added to the hole-transporter solution at 1:12 µl/mg of Li-TFSIsolution:spiro-MeOTAD. A small quantity (80 µl) of the spiro-OMeTADsolution was dispensed onto each perovskite coated film and spin-coatedat 1500 rpm for 30 s in air. The films were then placed in a thermalevaporator where 200 nm thick silver electrodes were deposited through ashadow mask under high vacuum (10⁻⁶ mBar).

Device Variations Investigated

A generic schematic of the device structure is shown in FIG. 1 a . Thisdevice can be mounted on any solid substrate material (glass, plastic,metal foil, metal mesh etc.). In FIG. 1 a at least one of the metallicelectrodes must be transparent/semitransparent (for example: doped orundoped metal oxide, perovskite, polymer, thin metal, metal mesh etc.)whereas the opposite electrode can be transparent/semitransparent orreflective. The light absorbing perovskite, which can be n-type, p-typeor intrinsic, is sandwiched between one n-type and one p-typesemiconducting layer (organic, inorganic, amorphous-Si, perovskite,hybrid organic/inorganic etc.) for selective electron and holeextraction respectively. The structure shown can be inverted.Multijunction cells can be produced by stacking a repeat structure.

Certain embodiments of the devices of the invention have the specificstructure shown in FIG. 1 b . When used, the thin metal oxide layer isgenerally permeable to the solution-processed perovskite, to ensuredirect contact of the perovskite with the electron-selective contact.Each of the preparation variations investigated here are summarised inTable 1.

TABLE 1 Summary of variations to layers investigated. TiO₂ compact layerheating conditions Mesoporous metal oxide paste Mesoporous oxidesintering conditions Perovskite deposition method Variation Label 150°C. Al₂O₃ with binder 500° C. Solution HT B-Al₂O₃ 150° C. TiO₂ withbinder 500° C. Solution HT B-TiO₂ 150° C. Al₂O₃ without binder 500° C.Solution HT Al₂O₃ 150° C. TiO₂ without binder 500° C. Solution HT TiO₂150° C. Al₂O₃ without binder 120° C. Solution LT Al₂O₃ 150° C. TiO₂without binder 120° C. Solution LT TiO₂ 150° C. + 500° C. Al₂O₃ withoutbinder 120° C. Solution HT C/LT Al₂O₃ 150° C. - - Solution LT C 150°C. + 500° C. - - Solution HT C 150° C. + 500° C. - - EvaporationEvaporated

Results and Discussion Porosity Control of Mesoporous Al₂O₃ Sintered atLow-Temperature

The porosity of a mesoporous layer of Al₂O₃ can be controlled by mixingtwo solvents with different viscosities and different evaporation ratesin the nanoparticle dispersion. After deposition from the dispersion andsolvent removal, the refractive index of a mesoporous compositethin-film of Al₂O₃ and air depends on the volume fraction of the twocomponents i.e. the porosity. The refractive indices of films formed byspin-coating dispersions with varying content of terpineol and t-butanolonto glass slides are presented in the Table 2 below, indicated asvolume equivalents. A lower refractive index is indicative of a largervolume fraction of air i.e. a more porous film. It is found in generalthat adding a secondary solvent increases the porosity of the resultingmesoporous film.

TABLE 2 Summary of porosity variation with a varying quantity of addedvolume equivalents of a secondary solvent into the alumina dispersion asindicated by the measured refractive index of resulting film. 20% by wtAl₂O₃ in IPA (equiv.) IPA added (equiv.) Terpineol added (equiv.)t-Butanol added (equiv.) Refractive Index 1 1 0 0 1.26 1 1 0.2 0 1.27 11 0.5 0 1.24 1 1 1 0 1.21 1 1 0 0.2 1.14 1 1 0 0.5 1.16 1 1 0 1 1.13

X-ray Diffraction

The XRD patterns of perovskite thin-films based on the differentunderlayer variations investigated are shown in FIG. 2 a . All sampleswere prepared on plain glass and, where thin mesoporous oxides arespecified, without compact layers. Both the 110 and 220 perovskite peaksare prominent in agreement with our previous demonstration of thisperovskite [Lee et al., Science, Submitted 2012]. FIG. 2 b shows the XRDpattern of the perovskite when evaporated. Peaks corresponding for themixed halide perovskite are present in addition to those occurring fromPbI₂.

UV-Vis Spectroscopy

The UV-vis patterns for perovskite thin-films based on the differentunderlayer variations investigated are shown in FIG. 3 . All sampleswere prepared on plain glass and, where thin mesoporous oxides arespecified, without compact layers. The spectra show the normalisedextinction (ε = log₁₀[I₀/I₁]). All spectra show an absorption onset at awavelength of ~800 nm confirming the presence of perovskite. AlthoughXRD diffraction peaks corresponding to PbI₂ are observed for theevaporated perovskite, the UV-vis spectrum indicates that most of thelight is absorbed by the perovskite. The shapes of the spectra are inagreement with our previous demonstration of this perovskite [M. Lee etal., Science, Submitted 2012].

Current-Voltage Characteristics

The current density-voltage (J-V) characteristics of some devicesrepresenting each variation investigated are presented in FIG. 4 . Asummary of the parameters extracted from these results is presented inTable 3. The thickness of the thin oxide layer (t_(mesoporous)) and theperovskite capping layer (t_(perovskite) _(cap)) as measured with asurface profilometer are also shown in Table 3. For thicknessmeasurements the samples were prepared on plain glass and, where thinmesoporous oxides are specified, without compact layers. The ratio ofthese thicknesses suggests that the majority of light absorption willoccur in the capping layer forming a planar heterojunction with the holetransport material.

TABLE 3 Summary of parameters extracted from J-V characteristics of mostefficient devices. Device Structure t_(mesoporous) (nm) t_(perovskite)_(cap) (nm) J_(sc) (mA/cm²) V_(oc) (V) Fill Factor PCE (%) HT B-Al₂O₃ 83221 14.69 0.95 0.30 4.24 HT B-TiO₂ 251 370 8.76 0.68 0.32 1.92 HT Al₂O₃91 659 11.60 0.92 0.29 3.14 HT TiO₂ 105 298 5.27 0.88 0.28 1.29 HT C -303 6.15 0.78 0.27 1.28 HT C/LT Al₂O₃ 72 407 15.2 0.92 0.32 4.55 LTAl₂O₃ 72 407 4.91 0.94 0.36 1.68 LT TiO₂ 104 350 1.68 0.9 0.60 1.19 LTC - 374 3.00 0.81 0.38 0.92 Evaporated - 100 - 300 5.33 0.73 0.21 0.83

Scanning Electron Microscopy

SEM micrographs of solar cell cross-sections are shown in FIG. 5(a)-(f).The distinct layers shown in the cross-sections are, from right to left:glass, FTO, compact layer, mesoporous layer, perovskite capping layer,spiro-OMeTAD and Ag. Planar images of mesoporous layers are shown inFIG. 6(a)-(f) and FIG. 7(a) and (b). Where Al₂O₃ is used, both with andwithout binder and sintered at both high and low temperatures, theimages clearly show a mesoporous structure allowing infiltration andseeding of the perovskite. Compact layers shown in FIG. 6(e) and 6(f)appear featureless at the resolution of the instrument. Where TiO₂ isused, with binder the film appears mesoporous. However, in the absenceof binder the nanoparticles aggregate forming a submonolayer.

Conclusion

The Examples show that it is possible to create planar n-type/perovskiteabsorber/p-type structured optoelectronic devices. Growth of aperovskite light absorber was achieved on a thin scaffold or in theabsence of scaffold from solution deposition. Devices incorporating athin seed layer can be processed entirely at temperatures not exceeding150° C. which is important for flexible and/or tandem/multijunctiondevices. Additionally it has been shown that the perovskite can byformed by evaporation from a bulk powder.

Inverted Heterojunction Perovskite Solar Cells Substrate Preparation:

Fluorine doped tin oxide (FTO) coated glass sheets (7 Ω/□ Pilkington)were etched with zinc powder and HCl (2 Molar) to obtain the requiredelectrode pattern. The sheets were then washed with soap (2% Hellmanexin water), de-ionized water, acetone, methanol and finally treated underan oxygen plasma for 5 minutes to remove the last traces of organicresidues.

TiOx Flat Film Precursor Solution

The TiO_(x) flat film precursor solution consists of 0.23 M titaniumisopropoxide (Sigma Aldrich, 99.999%) and 0.013 M HCl solution inethanol (>99.9% Fisher Chemicals). To prepare this solution, titaniumisopropoxide was diluted in ethanol at 0.46 M. Separately, a 2 M HClsolution was diluted down with ethanol to achieve a 0.026 Mconcentration. Finally, the acidic solution was added dropwise to thetitanium precursor solution under heavy stirring.

Regular Architecture Fabrication:

The etched FTO substrates were coated with a compact layer of TiO₂deposited by spin-coating the TiO_(x) flat film precursor solution at2000 rpm for 60s and consequently heating at 500° C. for 30 minutes toform stoichiometric anatase titania. Then the mesostructured scaffoldwas deposited by spin-coating a colloidal dispersion of ~20 nm Al₂O₃nanoparticles in isopropanol, followed by drying at 150° C. for 10minutes. After cooling down to room temperature, the perovskite wasdeposited by spin-coating from a DMF solution of methylammonium iodideand PbCl₂ (3:1 molar ratio), which formed the perovskite after heatingto 100° C. for 45 minutes. The hole-transport layer was deposited byspin-coating a 7 vol. % spiro-OMeTAD(2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene)in chlorobenzene solution with added 80 mM tert-butylpyridine (tBP) and25 mM lithium bis(trifluoromethanesulfonyl)Imide (LITFSI) at 1000 rpmfor 45 s. Finally, devices were completed with evaporation in a highvacuum of Ag contact electrodes through a shadow mask.

Inverted Architecture Fabrication: Pedot:pss:

The etched FTO substrates were coated with a thin film of PEDOT:PSSdeposited by spincoating a 25:75 vol% solution of PEDOT:PSS(Clevios):isopropanol (>99.9%, Fisher Chemicals) at 2000 rpm for 60 sand subsequently annealed at 150° C. for 20 minutes or crosslinked bysubmerging the substrates for 5 min in a 0.25 M FeCl₃ aqueous solution,subsequently washed in 2 sequential baths of deionized water thenfinally dried with nitrogen.

NiO:

The spincoating precursor for the NiO thin film was prepared bydissolving nickel acetate tetrahydrate and monoethanolamine in ethanolboth at a 0.1 M concentration under stirring in a sealed vial in air ona hotplate at 70C for 4h. The solution appeared homogenous and deepgreen.

V2o5:

The etched FTO substrates were coated with a thin film of V₂O₅ depositedby spincoating a 1:35 vol% solution of vanadium (V) oxitriisopropoxide(Sigma Aldrich) in isopropanol and subsequently heated to 500° C. toobtain crystalline vanadium oxide layers.

Perovskite and N-type Contact Deposition:

After cooling down/drying, the perovskite precursor solution wasspincoated at 2000 rpm for 20 s and it was then heated to 100° C. for 45minutes to form the structure. The electron selective contact wasdeposited by spincoating a 20 mgmL⁻¹ solution of [60]PCBM inchlorobenzene (Anhydrous, Sigma Aldrich) at 1000 rpm for 45 s. TheTiO_(x) flat film precursor solution was then spincoated at 3000 rpm for60 s and the films were annealed at 130° C. for 10 minutes. Finally,devices were completed with evaporation in a high vacuum of Al contactelectrodes through a shadow mask.

Results and Discussion

Perovskite-based thin film photovoltaic devices were recently reportedwith an architecture evolved from the solid-state dye-sensitized solarcells, where holes are collected through the metal cathode and electronsthrough the FTO anode (Ball, J. M.; Lee, M. M.; Hey, A.; Snaith, H.Low-Temperature Processed Mesosuperstructured to Thin-Film PerovskiteSolar Cells. Energy & Environmental Science 2013). In thisconfiguration, a thin film of mesoporous alumina is deposited overcompact TiO₂ covered FTO substrates to aid the perovskite filmformation, then an organic hole transporter is deposited over the formedstructure to provide a hole selective contact. However, since holes arecollected through the top metal cathode, this configuration has limitedapplications in tandem solar cells, where immediate improvements couldbe achieved by using the “wide bandgap” perovskites with a low band gapinorganic bottom cell, (Beiley, Z. M.; McGehee, M. D. Modeling low costhybrid tandem photovoltaics with the potential for efficienciesexceeding 20%. Energy & Environmental Science 2012, 5, 9173-9179) whichgenerally is constructed in a “substrate” configuration with electronsbeing collected at the top metal contacts.

Typical materials used in organic photovoltaics as hole selectivecontacts for the blends are PEDOT:PSS, V₂O₅ and NiO, while usuallyPC₆₀BM and more recently poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)](PFN) are used as electron acceptors. In order to determine whetherthese materials will work in the complete device, a good first step tocheck that charge transfer to these interlayers is possible is tomeasure the steady-state PL quenching efficiency, similarly to what hasbecome routine in all organic solar cells. This data is presented inFIG. 9 and the results are summarized in table 4. It can clearly be seenthat all the p-type layers chosen in this work quench the perovskite PLmore efficiently than the model spiro-OMeTAD system, with similar valuesfor PEDOT:PSS and V₂O₅ of 99.87 % quenching efficiency. All the n-typelayers exhibit significantly higher quenching rates than the model TiO₂system, which only exhibits a 45% steady-state quenching efficiency. Allthe cells fabricated in this work utilize a spin-coated PC₆₀BM layer asthe n-type contact, since solar cells fabricated with a PFN interlayeryielded an extremely poor photovoltaic performance.

TABLE 4 Steady-state Photoluminescence quenching efficiency for both pand n-type layers and the perovskite absorber. n-type layers p-typelayers NiO 95.39 PCBM 90.99 V2O5 99.87 PFN 99.86 PEDOT:PSS 99.86 TiO245.71 Spiro-OMeTAD 99.21

PEDOT:PSS as the P-type Contact

The first example of an inverted architecture using the perovskite asboth light absorber and charge transporter utilizes a thin PEDOT:PSSlayer as the p-type contact and a bi-layer of PC_(6o)BM and compactTiO_(x) as the n-type contact. To be able to process these structures inair, the top interlayer of TiO_(x) was necessary to achieve a goodcontact with the top Al anode. A cross-sectional SEM picture of theoptimized structure is presented in FIG. 10 . Uniform coverage of theperovskite structure is essential to fabricate optimal photovoltaicdevices and is heavily affected by the substrate it is formed on. Whenassembled on annealed PEDOT:PSS underlayers, macrocrystals of over 30 µmlength of perovskite are formed, as shown in FIG. 11 b) and 11 d). Whilethis might be beneficial for charge transport through the layer, ratherlarge micron-sized gaps exist between crystals which allow the directcontact between PC₆₀BM and the PEDOT:PSS underlayer, which is notbeneficial for device performance. PEDOT:PSS is soluble in DMF and forthis reason it has been crosslinked by submersion into a 0.25 M FeCl₃aqueous solution, to avoid redissolution of the layer when theperovskite precursor in DMF is deposited. When the PEDOT:PSS iscrosslinked, suprisingly the resulting perovskite film coverageincreases significantly while the average crystal/feature size for thismaterial has been reduced considerably. The resulting coverage andcrystal size is shown in FIG. 11 a) and 11 c), which was found to be 80± 1% for annealed PEDOT:PSS films and 97 ± 1% for crosslinked filmsestimated directly from the SEM images.

When the performance of the resulting devices is compared, as shown inFIG. 12 a ), it is found that the devices processed on crosslinkedPEDOT:PSS exhibit open circuit voltages of around 0.8 V, while deviceson annealed PEDOT:PSS only achieved about 0.64 V. This is consistentwith a reduction of charge recombination between charges in the PCBMlayer and the PEDOT:PSS layer due to the improved perovskite filmcoverage. Devices employing crosslinked PEDOT:PSS show a slightlyreduced short circuit current of 16.2 mAcm⁻ ² as compared to theannealed PEDOT:PSS devices which exhibit 17.2 mAcm⁻², although thedifference is small, and within the experimental variation. Finally, thepower conversion efficiency of the optimum devices reached values ofover 6.7%, severely outperforming annealed PEDOT:PSS devices whichreached 5.6%.

V₂O₅ and NiO as p-type Contacts

Both V₂O₅ and NiO are usual p-type materials currently in use for highefficiency and stable organic photovoltaic devices. Here, the inventorshave fabricated the devices by spincoating the appropriate precursorsolution on FTO with a subsequent sintering step at 500° C. to ensure afully crystalline metal oxide layer. Surface coverage of the perovskitesolution can be an issue for this materials, as can be seen in the SEMpictures of FIG. 13 .

The photovoltaic performance of devices incorporating these layers isshown in FIG. 14 .

Comparison With Regular Architectures

Finally, the champion inverted device, which is incorporates PEDOT:PSSas the hole accepting layer and PC₆₀BM as the electron extraction layer,is compared against a regular architecture device composed of a TiO₂electron accepting layer and spiro-OMeTAD as the hole transporting layerin FIG. 15 b ). Both systems achieve astounding short circuit currentsof over 17.5 mAcm⁻², and high open circuit voltages of over 0.9 V. Themain difference in power conversion efficiency of 11.8% for the regulararchitecture and 7.54% for the inverted devices, are the lower fillfactors of the latter. This is likely due to either leakage issuesbetween PEDOT:PSS and PCBM as shown in FIG. 5 .a. or series resistancelosses, likely due to the necessity of using a TiO_(x) overlayer to beable to process the devices in ambient air conditions.

The devices presented disclosed offer a completely new approach todesign architecture, particularly as the materials used are commonlyemployed and mass-produced at present for the organic photovoltaicsindustry and should therefore highly speed up the development of amass-producible system.

Conclusion

Inverted devices structures, where holes are collected through the FTO,are necessary for tandem applications for use with inorganicphotovoltaic bottom cells. Here is shown a low temperature, ambient airand solution-processable photovoltaic cell based on a semiconductingperovskite absorber and selective n-type and p-type contacts in the formof PEDOT:PSS and [60]PCBM. A 7.5% power conversion efficiency isachieved for these inverted structures. In one sense this demonstratesthe versatility of the perovskite thin-film technology to the broadvariety of possible device configurations, and equally importantly, thisremoves any barriers to adoption of the perovskite technology by theorganic photovoltaic community.

Two Source Vapour Deposition Substrate Preparation

The substrate preparation process was undertaken in air. Glass coatedwith fluorine-doped tin oxide (FTO) was patterned by etching with Znmetal powder and 2 M HCl diluted in milliQ water, and then it wascleaned with 2% solution of hellmanex diluted in milliQ water, rinsedwith milliQ water, acetone and ethanol and dried with clean dry air.Oxygen plasma was subsequently treated for 10 minutes. A compact layerof TiO₂ was spin-coated an acidic solution of titanium isopropoxide inethanol, and then sintered at 150° C. for 10 mins and then 500° C. for30 minutes.

Vapour Deposition

The system used is a dual-source evaporation to better manage theorganic source and inorganic source separately. The evaporator was theKurt J. Lesker Mini Spectros Deposition System with ceramic crucibles(OLED sources), housed in a nitrogen-filled dry glovebox (FIG. 18 ).Therefore, all the processes were operated in oxygen-free environment.The operation chamber is designed to work under a pressure of 5E-6mbarwhere the vapour particles are able to travel directly to the substrate.The samples were held face-down in a holder above crucibles containingthe source powders. Two crystal sensors monitor are located just 10 cmabove the crucibles to monitor the deposition rate of each sourceseparately without interference from each other. Their measurements areserved as a feedback to adjust the heating temperature to the sourcechemicals. Another crystal sensor is available near to the substrateholder that can be utilised to measure the total deposited thickness.

Tooling Factor Measurement

Since the source-to-monitor distance is different to thesource-to-substrate distance, the tooling factor (ratio of the materialsdeposited on the sensors to that on the samples) of each source wascalibrated individually. The density of CH₃NH₃I was assumed as 1 g/cm³due to its unavailability. The setting and results are shown in Table 5.

TABLE 5 Tooling factor measurement Setup Results Source DensityEvaporation rate Evaporation Time Values on sensors Real thickness inaverage Tooling factor Source 1: CH₃NH₃I 1 g/cm³ 16A/s 32 mins 32kA1.48um 2.16 Source 2: PbCl₂ 5.85 g/cm³ 5A/s 32 mins 10kA 185 nm 5.41

Note that it was hard to evaporate the organic source CH₃NH₃I constantlydue to its instability during the evaporating process and its depositionrate had up to +/-20% derivation from the set value. The physicalthickness was measured by Veeco DekTak 150 film thickness probe.

Dual Sources Pervoskite Deposition

The inventors aimed to investigate the ‘flat junction’ perovskite solarcells by evaporation in the dual source deposition system. Theevaporated perovskite can be deposited on the top of the compact layerof TiO₂ directly without mesoporous layer (FIGS. 20 b and 20 c ).

The organic source CH₃NH₃I and inorganic source PbCl₂ were weighedapproximately at 200 mg and 100 mg, and loaded to the two cruciblesrespectively. The samples were face-down inserted into the substrateholder. Once the pressure in the chamber was evacuated to 5E-6 mbar, theshutters of the two OLED sources were opened while heating up thesources. Once the two sources achieved the set values, the shutter ofthe substrate was opened with rotating the holder in order to obtain auniform thin film.

After completing the evaporation, the colour of the samples was changedcorresponding to the composition of the two sources. All the sampleswere then placed at hotplate to dry up at 100° C. for 50 mins tocrystallise perovskite crystal prior to spin-coat the hole-transporterlayer. FIG. 21 shows the surface image after annealing the perovskitecrystal on the hotplate. In the experiment so far, 7% spiro-OMeTAD inchlorobenzene solution with added tert-butylpyridine (tBP) and lithiumbis(trifluoromethanesulfonyl)imide (Li-TFSI) was used as thehole-transporter and was spin-coated at 2000 rpm for 45 s. At the end,devices were completed with evaporation of Ag contact electrodes (FIG.20 a ).

Comparing the evaporated perovskite with the conventional spin-coatedperovskite, the evaporated perovskite had more uniform and flat surfacewith less holes (FIG. 21 ). The full coverage of the evaporatedperovskite not only fully contacts with the compact layer of TiO₂, butalso isolates the hole-transport layer from the compact layer. It willcertainly help the photocurrent as well as the voltage throughout thesystem.

Device Characterisation

The experiment started with varying composition of CH₃NH₃I to PbCl₂ from4:1 to 16:1 in molar ratio under a constant total thickness. Once thecomposition had been optimised, the desired thickness under bestcomposition was investigated.

The best performance has been achieved to 13% electrical powerefficiency with setting the deposition rate to 5.3 Å/s for CH₃NH₃I and 1Å/s for PbCl₂ that ideally should have given 9.3:1 in molar ratio if thetooling factor is taken into consideration. However, as aforementioned,since the evaporation of the organic source CH₃NH₃I always hasdeviation, the final thickness shown on the Sensor 1 for CH₃NH₃I was44.4 kÅ instead of the expected 42.4 kÅ. In other words, the realaverage deposition rate for CH₃NH₃I should be 5.6 Å/s rather than thesetting value 5.3 Å/s. In this case, the film was actually depositedunder the molar ratio of CH₃NH₃I to PbCl₂ 9.8:1 which gave a physicalthickness of 230 nm measured by DekTak probe.

The best performance gave a short circuit photocurrent J_(sc) of21.47mA/cm2, open circuit voltage V_(oc) of 1.07 Volts and a fill factor(FF) of 0.67 yielding the power electrical conversion efficiency up to15.36% as shown in FIG. 22 . Current-Voltage characteristics weremeasured (2400 Series SourceMeter, Keithley Instruments) under simulatedAM 1.5G sunlight at 100 mW·cm⁻² irradiance and the solar cells weremasked with a metal aperture to define the active area which wastypically 0.076 cm² and measured in a light-tight sample holder tominimise any edge effects.

In order to define the contents in the evaporated film, XRD pattern ofthe evaporated perovskite surface was measured and then compared withthe conventional XRD pattern of the spin-coated perovskite and otheressential chemicals as shown in FIG. 23 . According to XRD patterns, itclearly shows that the evaporated perovskite is almost identical tothesolution processed perovskite film processed from CH₃NH₃I and PbCl₂precursors in DMF (termed K330) which indicates that the evaporatedperovskite has the same crystal structure as the spin-coated perovskite.

The last measurement in FIG. 24 was a comparison of the absorbancebetween 200 nm evaporated film and spin-coated film. The absorbance ofthe two 200 nm ‘flat-junction’ evaporated perovskite has the similarshape of the absorbance as the 200 nm ‘flat-junction’ spin-coatedperovskite, but the evaporated perovskite has much larger units inabsorption.

Conclusion

Here, evaporated hybrid inorganic-organic perovskite on flat-junctionsolar cells with over 15% electric power conversion efficiency aredemonstrated by appropriately controlling the deposition rate of theCH₃NH₃I and PbCl₂ and the deposited thickness on the substrate. Therealisation of utilising the evaporation technique to produce perovskitesolar cells overcomes the limits of solution process of finding asuitable solution to dissolve the chemicals and thus also helps thecommercialisation of the hybrid inorganic-organic solar cells.

In general, it is considered to be advantageous to retain a 3D crystalstructure in the perovskite, as opposed to creating layered perovskiteswhich will inevitably have larger exciton binding energies (Journal ofLuminescence 60&61 (1994) 269 274). It is also advantageous to be ableto tune the band gap of the perovskite. The band gap can be changed byeither changing the metal cations or halides, which directly influenceboth the electronic orbitals and the crystal structure. Alternatively,by changing the organic cation (for example from a methylammonium cationto a formamidinium cation), the crystal structure can be altered.However, in order to fit within the perovskite crystal, the followinggeometric condition must be met:

$\left( {R_{A} + R_{X}} \right) = t\sqrt{2}\left( {R_{B} + R_{X}} \right)$

wherein R_(A),_(B),_(&X) are the ionic radii of ABX ions. The inventorhave unexpectedly found that formamidinium cation (FO) does indeed formthe perovskite structure in a 3D crystal structure in a FOPbBr₃(tetragonal crystal) or FOPbI3 (cubic crystal) perovskite, and mixedhalide perovskites thereof.

Two Step Perovskite Layer Production Substrate Preparation

An electrode pattern was etched onto glass substrates coated withfluorine doped tin oxide (FTO, TEC 7 Pilkington Glass) using a mixtureof Zn powder and 2 M HCl. They were then sequentially cleaned inHallmanex, de-ionised water, acetone, propan-2-ol, and O₂ plasma.

Electron-selective Layer Deposition

A thin (approximately 50 nm) layer of TiO₂ served as theelectron-selective layer. It was deposited onto the substrate by spincoating (speed = 2000 rpm, acceleration = 2000 rpm/s, time = 60s) from afiltered solution (0.45 µm PTFE filter) containing Ti-isopropoxide inethanol with added HCl. These films were heated to 500° C. for 30minutes.

Evaporation of PbI₂ and PbCl₂

Thin-films (approximately 150 nm) of PbI₂ or PbCl₂ were deposited bythermal evaporation through a shadow mask onto the substrates at apressure of approximately 10⁻⁶ mbar at a rate of approximately 2 Å/s.The evaporation temperatures were approximately 270° C. and 310° C. forPbI₂ and PbCl₂ respectively.

Dip Coating Perovskite Conversion

For dip coating, substrates precoated in PbI₂ or PbCl₂ were immersedinto a 20 mg/ml solution of methylammonium iodide in anhydrouspropan-2-ol in a nitrogen filled glovebox. The dipping time was constantfor all devices at 10 minutes. After dip coating, the substrates wereannealed at 120° C. in a nitrogen atmosphere for 10 minutes. Dippingtimes may range from 10 seconds to 2 hours, For the example given inthis patent, the dipping time was 10 minutes.

Hole-transport Material Deposition

The hole-transport material,2,2′,7,7′-tetrakis-(N,N-di-pmethoxyphenylamine)9,9′-spirobifluorene(spiro-OMeTAD), was deposited by spin coating (speed = 2000 rpm,acceleration = 2000 rpm/s, time = 60s) from an 80 mM chlorobenzenesolution containing 80 mol% tert-butylpyridine (tBP) and 30 mol% lithiumbis(trifluoromethanesulfonyl)imide (Li-TFSI) as additives in a nitrogenfilled glovebox.

Top Electrode Deposition

The top silver electrode was deposited by thermal evaporation (pressureapproximately 5 µTorr) to a thickness of 150 nm at approximately 2 Å/s.

Device Current-voltage Characterisation

For measuring the performance of the solar cells, simulated AM 1.5sunlight was generated with a class AAB ABET solar simulator calibratedto give simulated AM 1.5, of 106.5 mW/cm² equivalent irradiance, usingan NREL-calibrated KG5 filtered silicon reference cell. The mismatchfactor was calculated to be 1.065 between 300 to 900 nm, which is beyondthe operating range of both the KG5 filtered silicon reference cell andthe perovskite test cells. The current-voltage curves were recorded witha sourcemeter (Keithley 2400, USA). The solar cells were masked with ametal aperture defining the active area (0.0625 cm²) of the solar cells.The current density-voltage characteristics of the devices are shown inFIG. 32 (for PbI₂ as the photoactive layer (dashed line) and CH₃NH₃PbI₃after dip coating as the photoactive layer (solid line)) and FIG. 33(for PbCl₂ as the photoactive layer (dashed line) andCH₃NH₃PbI_(3-x)Cl_(x) after dip coating as the photoactive layer (solidline)).

X-Ray Diffraction

X-ray diffraction (XRD) spectra were obtained from devices withoutsilver electrodes (FTO coated glass, TiO2, photoactive layer,spiro-OMeTAD) using a Panalytical X′Pert Pro x-ray diffractometer. Theresults are shown in FIG. 31 .

Scanning Electron Microscopy

Scanning electron microscopy (SEM) images were obtained from deviceswithout silver electrodes (FTO coated glass, TiO2, photoactive layer,spiro-OMeTAD) using a Hitachi S-4300. Electron micrographs are shown inFIG. 29 (for (a) PbCl₂ and (b) CH₃NH₃PbI₃₋ _(x)Cl_(x) after dip coating)and FIG. 30 (for (a) PbI₂ and (b) CH₃NH₃PbI₃ after dip coating).

Results and Discussion

The two step method allows for the production of uniform films ofperovskite using economical techniques which are already readilyavailable in the glazing industry. After an initial deposition of ametal dihalide, uniform planar films of perovskite may be produced byinfiltration of the metal dihalide with the organic halide. FIG. 31shows X-ray diffraction spectra of thin-films of (a) PbCl₂, (b)CH₃NH₃PbI_(3-x)Cl_(x), (c) PbI₂, and (d) CH₃NH₃PbI₃. After dip coating,films from both precursors show a relative intensity decrease of peakscorresponding to the precursor lattice and a relative increase of theperovskite lattice (absent in precursor xrd spectra) indicatingpredominant conversion of the precursor films into perovskite.

FIG. 29 shows cross-section scanning electron micrographs of devicesshowing, from bottom to top, the glass substrate, FTO, TiO₂electron-selective layer, photoactive layer, spiro-OMeTAD. Thephotoactive layers are (a) PbCl₂, and (b) CH₃NH₃PbI_(3-x)Cl_(x) afterdip coating. FIG. 30 shows cross-section scanning electron micrographsof devices showing, from bottom to top, the glass substrate, FTO, TiO₂electron-selective layer, photoactive layer, spiro-OMeTAD. Thephotoactive layers are (a) PbI2, and (b) CH₃NH₃PbI₃ after dip coating.In both instances, the perovskites produced by dip coating show relativeuniformity.

The Current density-voltage characteristics of the devices are shown inFIGS. 32 and 33 . In FIG. 32 , characteristics are shown for a devicemade using PbI₂ as the active layer (dashed line) and a device where theevaporated PbI₂ has been converted to CH₃NH₃PbI₃ (solid line) by dipcoating in a methylammonium iodide solution in propan-2-ol. Theperformance parameters for the PbI₂ are J_(sc) = 1.6 mA/cm², PCE =0.80%, V_(oc) = 0.97 V, FF = 0.57. The performance parameters for theCH₃NH₃PbI₃ are J_(sc) = 5.3 mA/cm², PCE = 2.4%, V_(oc) = 0.82 V, FF =0.61. In FIG. 33 current density-voltage characteristics of a devicemade using PbCl₂ as the active layer (dashed line) and a device wherethe evaporated PbCl₂ has been converted to CH₃NH₃PbI_(3-x)Cl_(x) (solidline) by dip coating in a methylammonium iodide solution in propan-2-olare shown. The performance parameters for the PbCl₂ are J_(sc) = 0.081mA/cm², PCE = 0.006 %, V_(oc) = 0.29 V, FF = 0.27. The performanceparameters for the CH₃NH₃PbI_(3-x)Cl_(x) are J_(sc) = 19.0 mA/cm², PCE =7.0%, V_(oc) = 0.8 V, FF = 0.49. In both cases, it is shown that viabledevices are produced by this two step method.

Charge Carrier Diffusion Length Estimation

For a charge (either electron or hole) to be generated from lightabsorption and collected efficiently from a thin solid film, thelifetime of the charge species (the time it will live for beforerecombining with an oppositely charged species) must be longer than thetime it takes to diffuse across the film and flow into the electrode.The product of the diffusion coefficient (D_(e)) and the lifetime(τ_(e)) can be used to estimate the diffusion length (L_(D)) following

$L_{D} = \sqrt{D\tau}.$

Photoluminescence (PL) quenching has been previously employedsuccessfully with organic semiconductors, in order to determine thediffusion length of the photoexcited bound electron-hole pair (theexciton). By simply fabricating solid thin films in the presence orabsence of an exciton quenching layer, and modelling thephotoluminescence decay to a diffusion equation, it is possible toaccurately determine the exciton lifetime, diffusion rate and diffusionlength. A cross-sectional SEM image of a 270-nm thick mixed halideabsorber layer with a top hole-quenching layer of Spiro-OMeTAD is shownin FIG. 36 .

The PL decay dynamics were modelled by calculating the number anddistribution of excitations in the film n(x,t) according to the 1-Ddiffusion equation (eq. 1),

$\frac{\partial n\left( {x,t} \right)}{\partial t} = D\frac{\partial^{2}n\left( {x,t} \right)}{\partial x^{2}} - k(t)n\left( {x,t} \right)$

where D is the diffusion coefficient and k(t) is the PL decay rate inthe absence of any quencher material. The total decay rate k wasdetermined by fitting a stretched exponential decay to the PL datameasured from perovskite layers coated with PMMA. The effect of thequencher-layer was included by assuming that all photogenerated carrierswhich reach the interface are quenched, giving the boundary conditionn(L,t)=0, where x=0 at the glass/perovskite interface and L is theperovskite film thickness. As the samples were photo-excited from theglass substrate side of the samples, the initial distribution ofphotoexcitations was given by n(x,0)=n₀exp(-αx), where α is theabsorption coefficient. The diffusion length L_(D) of the species wasthen determined from

$L_{D} = \sqrt{D\tau},$

where τ=⅟k is the recombination lifetime in the absence of a quencher.If free charges are predominantly created upon photoexcitation, the PLdecay represents the depopulation of charge carriers and the diffusioncoefficients for holes or electrons can be estimated depending on whichquenching layer is employed. The results from the diffusion model fitsare shown in FIG. 34 and FIG. 35 . and the parameters summarized inTable 6 (D is the diffusion constant and L_(D) the diffusion length).

TABLE 6 Perovskite Species D (cm²s⁻¹) L_(D)(nm) CH₃NH₃PbI_(3-x)Cl_(x)Electrons 0.042 ± 0.016 1094 ± 210 Holes 0.054 ± 0.022 1242 ± 250CH₃NH₃PbI₃ Electrons 0.017 ± 0.011 117 ± 38 Holes 0.011 ± 0.007 96 ± 29

Triiodide (CH₃NH₃PbI₃) and mixed halide (CH₃NH₃PbI_(3-x)Cl_(x))perovskites are compared in FIG. 37 which shows photoluminescence decayfor a mixed halide organolead trihalide perovskite filmCH₃NH₃PbI_(3-x)Cl_(x) (Black squares) and an organolead triiodideperovskite film CH₃NH₃PbI₃ (grey squares), coated with PMMA. Lifetimesτ_(e) quoted as the time taken to reach ⅟e of the initial intensity.

Strikingly, the diffusion lengths for both electrons and holes in themixed halide perovskite are greater than 1 µm, which is significantlylonger than the absorption depth of 100 to 200 nm. This indicates thatthere should be no requirement for meso or nanostructure with thisspecific perovskite absorber. The triiodide perovskite CH₃NH₃PbI₃ filmshave shorter diffusion length of around 100 nm for both electrons andholes. The large diffusion lengths of the mixed halide perovskite allowsphotovoltaic devices with layers of perovskite with thicknesses inexcess of 100 nm to be constructed which show excellent devicecharacteristics.

Methods Perovskite Precursor Preparation:

Methylamine iodide (MAI) was prepared by reacting methylamine, 33 wt% inethanol (Sigma-Aldrich), with hydroiodic acid (HI) 57 wt% in water(Sigma-Aldrich), at room temperature. HI was added dropwise whilestirring. Upon drying at 100° C., a white powder was formed, which wasdried overnight in a vacuum oven and recrystallized from ethanol beforeuse. To form the CH₃NH₃PbI_(3-x)Cl_(x) or CH₃NH₃PbI₃ precursor solution,methylammonium iodide and either lead (II) chloride (Sigma-Aldrich) orlead (II) iodide (Sigma-Aldrich) were dissolved in anhydrousN,N-Dimethylformamide (DMF) at a 3:1 molar ratio of MAI to PbCl₂/PbI₂,with final concentrations 0.88 M lead chloride/iodide and 2.64 Mmethylammonium iodide.

Substrate Preparation:

Glass substrates for absorption, TA and PL measurements were cleanedsequentially in 2% hallmanex detergent, acetone, propan-2-ol and oxygenplasma. Devices were fabricated on fluorine-doped tin oxide (FTO) coatedglass (Pilkington, 7 Ω □⁻¹). Initially FTO was removed from regionsunder the anode contact, to prevent shunting upon contact withmeasurement pins, by etching the FTO with 2 M HCl and zinc powder.Substrates were then cleaned and plasma-etched as above. A hole-blockinglayer of compact TiO₂ was deposited by spin-coating a mildly acidicsolution of titanium isopropoxide in ethanol, and annealed at 500° C.for 30 minutes. Spin-coating was carried out at 2000 rpm for 60 seconds.

Perovskite Deposition:

To form the perovskite layer for spectroscopy measurements, thenon-stoichiometric precursor was spin-coated on the substrate at 2000rpm in air. For CH₃NH₃PbI_(3-x)Cl_(x,) the precursor was used as is; forthe CH₃NH₃PbI₃, the precursor was diluted in DMF at a 1:1 ratio ofprecursor solution to DMF. After spin-coating, the CH₃NH₃PbI_(3-x)Cl_(x)films were annealed at 100° C. for 45 minutes, and the CH₃NH₃PbI₃ at150° C. for 15 minutes. The top quenchers were then deposited in air viaspin-coating chlorobenzene solutions with the following conditions:poly(methylmethacrylate) (PMMA; Sigma-Aldrich) at 10 mg/ml andphenyl-C₆₁-butyric acid methyl ester (PCBM; Solenne BV) at 30 mg/ml,both spin-coated at 1000 rpm, and2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene(spiro-OMeTAD; Borun Chemicals) at 0.46 M spin-coated at 2000 rpm.

Characterization:

A field emission scanning electron microscope (Hitachi S-4300) was usedto acquire SEM images. Sample thicknesses were measured using a VeecoDektak 150 surface profilometer.

Photoluminescence Measurements and Fits:

Steady-state and time-resolved PL measurements were acquired using atime-correlated single photon counting (TCSPC) setup (FluoTime 300,PicoQuant GmbH). Film samples were photoexcited using a 507 nm laserhead (LDH-P-C-510, PicoQuant GmbH) pulsed at frequencies between 0.3-10MHz, with a pulse duration of 117ps and fluence of ~30nJ/cm². The PL wascollected using a high resolution monochromator and hybridphotomultiplier detector assembly (PMA Hybrid 40, PicoQuant GmbH).

Parameters describing the photoluminescence dynamics in the absence ofany quencher are required inputs in the diffusion model. These wereobtained by fitting the background-corrected PL measured fromPMMA-capped perovskite films with a stretched exponential decay functionof the form,

I(t) = I₀e^(−(t/τ_(e))^(β))

Errors in the fitting parameters were determined by examining thereduced χ² surfaces obtained by independently varying each fittingparameter. A cut-off value of χ_(R) ²(p)/ χ_(R) ²= 1.2 was used in eachcase to obtain limits at a confidence level of 68 %. For ease ofcomparison of lifetimes between samples with different quenchers, τ_(e)is defined as the time taken after excitation for the PL intensity todrop to ⅟e of its peak intensity. The error in the accuracy of thislifetime was taken to be the half of the range of points whose meanvalue lies within one standard deviation of the ⅟e line. Results ofphotoluminescence dynamics are shown in FIGS. 34, 35 and 37 .

Diffusion Modeling:

The PL decay dynamics was modeled by calculating the number anddistribution of excitations in the film n(x,t) according to the 1-Ddiffusion equation,

$\frac{\partial n\left( {x,t} \right)}{\partial t} = D\frac{\partial^{2}n\left( {x,t} \right)}{\partial x^{2}} - k(t)n\left( {x,t} \right)$

where D is the diffusion coefficient and k(t) is the PL decay rate inthe absence of any quencher material. The total decay rate, k = ⅟k_(f) +⅟k_(nr) = βτ^(-β)t^(β-1), was determined by fitting a stretchedexponential decay to the PL data measured from perovskite layers withPMMA and assumed independent of the capping material. The effect of thequenching layer was included by assuming that all excitons which reachthe interface are quenched with unit efficiency (n(L,t) =0, where x=0 atthe glass/perovskite interface and L is the perovskite film thickness).As the excitation pulse was from the glass substrate side of thesamples, the initial distribution of excitons was taken to ben(x,0)=n₀exp(-αx), where α=A/L (absorbance at 507 nm / perovskite layerthickness). Any deviation from this distribution due to reflection ofthe laser pulse at the perovskite/quencher interface was assumed to benegligible. In order to calculate the diffusion length L_(D), thediffusion coefficient was varied to minimize the reduced chi-squaredvalue,

$\chi_{r}^{2} = \frac{1}{\left( {n - p - 1} \right)}{\sum\frac{\left( {y(t) - y_{0}(t)} \right)^{2}}{y(t)}}$

where y(t) and y_(c)(t) are the measured and calculated PL intensitiesat time t, n is the number of data points and p is the number of fittingparameters. The equation was solved numerically using theCrank-Nicholson algorithm and the number of excitons integrated acrossthe entire film in order to determine the total PL intensity at time t.Both the stretched exponential and 1-D diffusion models were fit to theexperimental TCSPC data by iterative reconvolution with the instrumentresponse function (IRF) which was recorded separately, such that theobserved PL intensity,

I(t) = ∫g(t)f(t − t^(′))dt^(′)

is the result of the real decay curve, ƒ(t), convolved with the IRF,g(t). The average diffusion length L_(D) is given by

$L_{D} = \sqrt{D\tau_{s}},$

where τ_(ε) is the time taken for the PL to fall to ⅟e of its initialintensity in the absence of any quencher.

1-105. (canceled)
 106. A photovoltaic device which is a tandem junctionphotovoltaic device or a multi junction photovoltaic device, wherein thephotovoltaic device comprises a photoactive region which comprises: ann-type region comprising at least one n-type layer; a p-type regioncomprising at least one p-type layer; and, disposed between the n-typeregion and the p-type region: a layer of a perovskite semiconductorwithout open porosity.
 107. A photovoltaic device according to claim 106which comprises a first electrode, a second electrode, and, disposedbetween the first and second electrodes: (i) said photoactive region;and (ii) at least one other photoactive region.
 108. A photovoltaicdevice according to claim 106 wherein the thickness of the layer of theperovskite semiconductor without open porosity is from 10 nm to 100 µm.109. A photovoltaic device according to claim 106 wherein the thicknessof the layer of the perovskite semiconductor without open porosity isfrom 100 nm to 100 µm.
 110. A photovoltaic device according to claim 106wherein the perovskite semiconductor has a three-dimensional crystalstructure.
 111. A photovoltaic device according to claim 106 wherein thelayer of the perovskite semiconductor without open porosity forms afirst planar heterojunction with the n-type region and a second planarheterojunction with the p-type region.
 112. A photovoltaic deviceaccording to claim 106 wherein the photoactive region comprises: saidn-type region; said p-type region; and, disposed between the n-typeregion and the p-type region: (i) a first layer which comprises a porousscaffold material and a perovskite semiconductor disposed in pores ofthe scaffold material, optionally wherein the scaffold material is adielectric scaffold material or a charge-transporting scaffold material;and (ii) a capping layer disposed on said first layer, which cappinglayer is said layer of a perovskite semiconductor without open porosity,wherein the perovskite semiconductor in the capping layer is in contactwith the perovskite semiconductor in the first layer.
 113. Aphotovoltaic device according to claim 106 wherein the perovskitesemiconductor comprises at least one anion selected from halide anions.114. A photovoltaic device according to claim 113 wherein the perovskitecomprises a first cation which is an organic cation, a second cationwhich is a metal cation, said at least one anion selected from halideanions, and optionally further cations or further anions.
 115. Aphotovoltaic device according to claim 114 wherein the first cation is(H₂N═CH—NH₂)⁺ and the second cation is Pb²⁺ or Sn²⁺.
 116. A photovoltaicdevice according to claim 107 wherein the at least one other photoactiveregion comprises at least one layer of a semiconductor material.
 117. Aphotovoltaic device according to claim 116 wherein the semiconductormaterial comprises a layer of crystalline silicon, copper zinc tinsulphide, copper zinc tin selenide, copper zinc tin selenide sulphide,copper indium gallium selenide, copper indium gallium diselenide orcopper indium selenide.
 118. A photovoltaic device according to claim116 wherein the at least one layer of a semiconductor material comprisesa layer of crystalline silicon.
 119. A photovoltaic device according toclaim 116 wherein the semiconductor material comprises a perovskitesemiconductor.
 120. A photovoltaic device according to claim 116 whereinthe semiconductor material comprises an organic semiconductor.
 121. Aphotovoltaic device according to claim 107 wherein the at least oneother photoactive region comprises a crystalline silicon Heterojunctionwith Intrinsic Thin layer (c-Si HIT) cell.
 122. A photovoltaic deviceaccording to claim 106, wherein: (A) the photovoltaic device comprisesthe following regions in the following order: I. a first electrode; II.said photoactive region; III. a layer (A) of a p-type semiconductor,optionally p-type amorphous silicon; IV. a first layer of an intrinsicsemiconductor, optionally intrinsic amorphous silicon; V. a layer (B) ofa p-type semiconductor or a layer (B) of an n-type semiconductor,optionally a layer of p-type crystalline silicon or a layer of n-typecrystalline silicon; VI. a second layer of an intrinsic semiconductor,optionally intrinsic amorphous silicon; VII. a layer (C) of an n-typesemiconductor, optionally n-type amorphous silicon; and VIII. a secondelectrode; or (B) the photovoltaic device comprises the followingregions in the following order: I. a first electrode; II. saidphotoactive region; III. a layer of a transparent conducting oxide; IV.a layer (D) of an n-type semiconductor, optionally wherein the n-typesemiconductor is a metal oxide or metal chalcogenide; V. a layer ofcopper zinc tin sulphide, copper zinc tin selenide, copper zinc tinselenide sulphide, copper indium gallium selenide, copper indium galliumdiselenide or copper indium selenide; and VI. a second electrode; or (C)the photovoltaic device comprises a first electrode, a second electrode,and, disposed between the first and second electrodes: said photoactiveregion and at least one other photoactive region, wherein each of the atleast one other photoactive regions comprises: an n-type regioncomprising at least one n-type layer; a p-type region comprising atleast one p-type layer; and, disposed between the n-type region and thep-type region: a layer of a perovskite semiconductor without openporosity.
 123. A process for producing a photovoltaic device which is atandem junction or multi junction photovoltaic device, wherein thephotovoltaic device comprises a photoactive region which comprises: ann-type region comprising at least one n-type layer; a p-type regioncomprising at least one p-type layer; and, disposed between the n-typeregion and the p-type region: a layer of a perovskite semiconductorwithout open porosity, which process comprises: (a) providing a firstregion; (b) disposing a second region on the first region, which secondregion comprises the layer of a perovskite semiconductor without openporosity; and (c) disposing a third region on the second region,wherein: the first region is said n-type region comprising at least onen-type layer and the third region is said p-type region comprising atleast one p-type layer; or the first region is said p-type regioncomprising at least one p-type layer and the third region is said n-typeregion comprising at least one n-type layer.
 124. A process according toclaim 123 which further comprises: (d) disposing a tunnel junction onthe third region; (e) disposing a further photoactive region on thetunnel junction, which is the same as or different from the photoactiveregion defined in claim 18; (f) optionally repeating steps (d) and (e);and (g) disposing a second electrode on the further photoactive regiondisposed in the preceding step.
 125. A process according to claim 124wherein the further photoactive region comprises at least one layer of asemiconductor material, wherein the at least one layer of asemiconductor material comprises a layer of crystalline silicon, copperzinc tin sulphide, copper zinc tin selenide, copper zinc tin selenidesulphide, copper indium gallium selenide, copper indium galliumdiselenide or copper indium selenide, a perovskite semiconductor, or anorganic semiconductor.